Alaska cruises are scenic and absolutely addictive but you really should rethink a Seattle round trip. You will spend two days at sea and sail via the Pacific while only visiting 4 or 5 ports. Be sure to compare the time you will spend in each port if you are also comparing HAL ships or other lines. For many excursions you need 6 hours plus cushion times.
You mentioned sea sickness but have never cruised on an ocean going vessel before. There are remedies like Bonine that will have no side effects or the scopolomine patch that does have side effects but generally deal with a little mal de mer.
What others have not mentioned is timing. Early or late in the season it is possible to encounter fewer ships which is a real bonus. May and September are months to consider. Alaska is in the eco rain forest zone and it is best to prepare for cooler weather in those months but you can experience rain at any time.
I was thinking about excursions and for Alaska I'd average the cost to about $100 per. That would cover the low and high. Five ports = $500 per person.
Cruise Critic is a wonderful resource for all cruise lines. The Ports of Call boards will provide vetted information and opinion about Alaska. The Cruise Line boards will list the lines and ships. The most important board for assistance with planning is the Roll Call board. You find your cruise line, click on the specific ship and then find the date for your cruise.
Unless you can find a longer cruise, and there are some, I still recommend you sail from ports other than Seattle. I have sailed on HAL, and I have sailed in a suite or two and I would not book a suite for a week's Seattle round trip, but that's just me.
Consult a travel agent, preferably one you can sit down with to plan your FIRST cruise. A professional can decipher the variety of cabin options and decks and so much more.
I hope you do find a cruise that works. Enjoy your first cruise to Alaska.