Welcomw to Cruise @ddicts!!
My DH also has Parkinson's -- has gotten a little worse this past year because of some personal issues at home. But he isn't as bad as your grandmother from you have described.
Keep in mind that Alaskan cruises that sail out of Seattle do not take the inside passage -- they head for the Pacific which can be rough at times until you hit the first port of call. And then rough as you head back to Seattle on the last sea day. The Inside Passages are the best for ship not to rock much -- but these cruises sail out of Vancouver.
HAL has some type of show 6 nights of a 7 day cruise -- there will be 2 production shows. We raely go to them anymore as they are repeat shows for us. Other than the production shows, there will be comedians, jugglers, singers, etc -- varies by ship.
On HAL your choices of cruises for 7 days out of Seatte are Oosterdam, Zaandam and Rotterdam.
You should really consider 3 people to a cabin -- balcony cabins can handle this -- otherwise you will find that HAL will charge anywhere from 150% to 200% for a cabin for a solo cruiser.
You didn't mention if your grandmother needs to have a handicapped cabin -- this could also be a factor in what type of cabin you book.
Here is a link to HAL's site for handicapped people:
Cruise Planning & Advice, Preparation | Cruise Tips and Info | Holland America Line
The first couple of weeks in August will be the warmest -- especially the first week.. Among the various cruises that we have done in Alaska, we did cruise one year at the end of Augusy and the first part of September -- had a lot of rain.
