kadi,
While your request at first looks pretty simple, I was surprised to see how few cruises came close to your ruquest.
The first thing I did was to look for cruises on the lines that I know have water slides on many of their ships: Carnival, RCCL, and NCL. Then I narrowed the search from there.
RCCL has Seranade of the Seas with a water slide doing Round Trip Circle Hawaii, but out of Los Angles not Vancouver.
Carnival Spirit, with water slide has 10 day one way cruises, which could get you your round trip if you were able to do back to back, but it looked like the return cruise ends in Ensenada.
NCL has water slides on many of their ships, but they only serve Hawaii with 7 day RT from Honolulu.
You may have to make a decision about what part of your criteria is the most important, Departure Port, Destination, Round Trip, or Water Slide. If you can be more flexible on those points then you have many more options.
My suggestions would be to perhaps forego the water slide. The crossing to Hawaii is likely to be cool and the kids may have a better time indoors on a line that is know for their great kids program.
If you have a good travel agent, he or she can give you some more advice. You won't pay anymore to book through an agent, and you'll get thier insight as well.