There is nothing sailing to Alaska in April. It's too early in the season but I would agree with Mike. These are really calm seas. That being said, ocean rules are broken all the time. One of the roughest cruises I've ever been on was in the Western Caribbean.
I don't know where your fear lies, but if it's about getting seasick, I would advise traveling with the Patch and getting some Bonine (over the counter and cheap) as well. Between the two they will stop any motion sickness right away, before it begins. If you're afraid of the ship's safety, any cruiseline sailing out of US waters must be in compliance with SOLAS (safety of life at sea) standards. It isn't that ships can't sink, it's that they don't. If they did, their Global Positioning System would notify the nearest coast guard and/or navy where they were within a 90 ft. radius within 2 minutes. They carry twice as many lifeboats as people onboard and they are just a whole lot more careful than they were in the old days. Cruiselines don't want to lose their passengers and crew or their multi million dollar babies either. I hope you have a wonderful cruise and relax into what we all love. Hope you do too.
You may also want to look at the RCCL's Voyager class ships, they are so big, you hardly feel much movement at all. There are 3 now to choose from doing Eastern, Western and Southern. Also having a window or balcony will help settle your stomach, and I also travel with bonnine and so far have never needed it.
Are there any specific islands you want to see, may-be start there and then find the ships that go there, good luck and have fun!
calmest sea is eastern, werstern caribbean. Ive been on almost 300 cruises and thats the smoothest, howver u can feel movemment anywhere depends on wind direction and air currents. best cabin as mentioned is lower down center, DONT get window as u will feel motion by looking out. Also size of ship DOESNOT make it much smoother, ocean is very big any ship in rough weather , you will feel movement.
Might try a three or four day Bahamas cruise out of MIA or FLL. The islands provide a sea break and are so close the ship doesn't have to make any way to get there on time. Odds of calm (boringly smooth, like are we really at sea?) sea state are best.