Rough seas are mostly caused by high winds from storms. Stormy weather can occur at any time, on any day. I suggest checking with any weather web site for Hawaii climate, and schedule your cruise during a dry season.
Here's one that might help:
http://www.wrcc.dri.edu/narratives/HAWAII.htm
CLIMATIC REGIONS OF HAWAII – It is convenient to recognize seven climatic sub regions in Hawaii. These are defined chiefly by the major physiographic features of the State and by location with reference to windward or leeward exposure. Since one region grades into another, it would be misleading to attempt to draw sharp boundaries between adjacent regions. In general, however, the regions and their characteristics are as follows:
(1) WINDWARD LOWLANDS, generally below 2,000 feet on the north to northeast sides of the islands. This region lies more or less perpendicular to the prevailing flow of the trade winds, and is moderately rainy, with frequent trade wind showers. Partly cloudy to cloudy days are common. Temperatures are more nearly uniform and mild than in other regions.
(2) LEEWARD LOWLANDS, except for the Kona coast of Hawaii which has a distinctive climate. In these areas daytime temperatures are slightly higher and nighttime temperatures are slightly lower than in windward locations. Dry weather prevails except for occasional light trade wind showers which drift over from the mountains to windward and for periods of major storms. In some leeward areas an afternoon sea breeze in common, especially in summer
(3) INTERIOR LOWLANDS, on Oahu and Maui. In the northeast these lowlands have the character of the windward lowlands; in the southwest, of leeward lowlands. The central areas are intermediate in character, and – especially on Oahu – are sometimes the scene of intense local afternoon showers from well-developed clouds which form as a result of local heating of the land during the day.
(4) THE KONA COAST OF HAWAII. This is the only region in the islands where summer rainfall exceeds winter rainfall. There is a marked diurnal wind regime, with well-developed and reliable land and sea breezes, especially in the summer. Summer is also the season with a high frequency of late afternoon or early evening showers. Conditions are somewhat warmer and decidedly drier than in windward locations.
(5) RAINY MOUNTAIN SLOPES ON LEEWARD SIDE. Rainfall and cloudiness are very high, with considerable rain both winter and summer. Temperatures are equable. Humidities are higher than in any other region.
(6) LOWER MOUNTAIN SLOPES ON LEEWARD SIDE. Rainfall is greater than on the adjacent leeward lowlands, but distinctly less than at the same level on the windward side except that the zone of maximum rainfall usually occurs just to leeward of the crests of the lower mountains. Temperature extremes are greater than on the rainy slopes of the windward sides of the mountains, and cloudiness is almost as great.
(7) HIGH MOUNTAINS. Above 2,000 or 3,000 feet on the high mountains of Mauna Kea, Mauna Loa, and Haleakala rainfall decreases rapidly with elevation. Near the summits of Mauna Loa and Mauna Kea, rainfall is scant and skies are clear a high percentage of the time. Relative humidities may reach values of ten percent of less. The lowest temperatures in the State are experienced in this region, with values below freezing being common.
THE CLIMATIC SETTING – The most prominent feature of the circulation of air across the tropical Pacific is the persistent trade-wind flow in a general east-to-west direction. In the central North Pacific the trade winds blow from the northeast quadrant, and represent the outflow of air from the great region of high pressure, the Pacific Anticyclone, whose typical location is well north and east of the Hawaiian Island Chain. The Pacific High, and with it the trade-wind zone, moves north and south with the sun, so that it reaches its northern-most position in the summer half-year. This brings the heart of the trade winds across Hawaii during the period May through September when the trades are prevalent 80 to 95 percent of the time. From October through April, Hawaii is located to the north of the heart of the trade winds. Nevertheless, the trades still blow across the islands much of the time, though with a frequency that has decreased to 50 to 80 percent in terms of average monthly values.
The dominance of the trades and the influence of terrain give special character to the climate of the islands. Completely cloudless skies are extremely rare, even though much of the time the dense cloud cover is confined to the mountain areas and windward slopes, while the leeward lowlands have only a few scattered clouds. Showers are very common; yet while some of these are very heavy, the vast majority are light and brief – a sudden sprinkle of rain and that is all. Even the heavy showers are of a special character, in that they are seldom accompanied by thunder and lightening. Indeed, a flash of lightening followed by a crash of thunder is infrequent enough so that many people who have lived only in Hawaii have no real notion of the violence of Mainland thunderstorms as evidenced by the lightning and thunder that are their typical accompaniment. Finally, the trade winds provide a system of natural ventilation much of time throughout most of the State and bring to the land, at least in the lower laying regions, the mildly warm temperatures that are characteristic of air that has moved great distances across the tropical seas.
But the trades, though dominant, are not the only major dynamic element in the climatic setting of Hawaii. Major storm systems may influence all parts of the islands, and in some areas there is a local exchange of air between land and sea on a rhythmic, diurnal basis. There is also the fundamental diurnal cycle of night and day, and because Hawaii is in such a low latitude this cycle shows less variation in length than is found in any other state.
Major storms occur most frequently between October and March, inclusive. During this period there may be two, three or even as many as six or seven major storm events in any particular year. Such storms typically bring heavy rains; and are sometimes accompanied by strong winds, at least on a local scale. The storms may be associated with the passage of a cold front – the leading edge of a mass of relatively cool air that is moving from west to east of from northwest to southeast. The storms may also be associated with a large eddy, or Low, that has been generated in the moving air in somewhat the same manner that an eddy is generated in a flowing river. Moist, warm air swirling into such eddies produces tremendous clouds and torrential rains.
Looks like April to Sepetember would be a great time to schedule a Hawaiian cruise if you wish to avoid storms...... Shucks, anytime would be a great time!
