I haven't done the Hawaii + Mexico, but I have done the 15 days San Diego to Hawaii and back, with just a brief stop in Mexico. I was on the Zaandam. I've also done a 16-day transatlantic and will be taking a 17-day one next month.
My husband, who hates sea days because all he does is sit somewhere and read, didn't come; but men can easily get through the formal nights with just a dark suit. Maybe change ties. I have two VERY simple but long dresses that take up very little room in my suitcase; if I need more, I also have a long black skirt and some relatively dressy tops. I only bring one pair of dressy shoes.
I love sea days, because I participate in all the games and activities (except bingo). The transit to Hawaii is usually four days, and HAL provides a good assortment of daily activities, from games and sports contests to lectures and movies in the theater. It is not unusual to have to choose among them because you can't make it to everything, and that's not even allowing for pool time. If there's an activity you particularly love, talk to the cruise director about it. On my prior transatlantic someone asked that a get-together for needleworkers be listed in the daily activities, and close to 30 people showed up for it.
And not only is it not disconcerting to not see land, it can be very pleasant just to watch the ocean. Coming back from Hawaii our ship met the largest pod of dolphins I've ever seen, literally hundreds of them. The group divided as we approached and swam by on both sides of the ship, with groups of four or five of them leaping in parallel at the same moment. Absolutely fantastic to see.