Unfortunately, you will not find a repositioning cruise in July. Repositioning cruises take place only when cruiselines move their ships from one itinerary to another at the beginning/end of a cruise season. Transatlantic cruises are, in fact, repositioning cruises when ships move from the Caribbean to Europe in late spring for the summer season, and then back again in the fall. Similarly, ships are moved to/from the Caribbean for the Bermuda, Alaska and trans-canal run, again sometime in the late spring and early fall.
Re Alaska cruises, there are roundtrip loops out of Vancouver and one-way northbound/southbound cruises out of Seward. Tavel to and from Vancouver is easier than travel to/from either Fairbanks or Anchorage, but if a trip to Denali is in the offing, you'll need to look at one-way cruises only. Some itineraires include Glacier Bay, while others visit Hubbard Glacier instead. While both are impressive, Glacier Bay is a must see in my opinion. Weather in Alaska is, in a word, unpredictable. We've done five Alaska cruises and have had weather that was warm and sunny, cool and rainy, and various combinations thereof.
The choice between RCI and Celebrity is a personal one. We've done many cruises, including Alaska, on both lines. Celebrity is our favorite, but I don't thing you can go wrong with RCI either.
