Hi, I did the Atlantis Submarine tour in Hawaii last November and it was incredible. I suspect all of them would be similar just in different locations. I am not a swimmer and tried snorkeling while I was in Cuba. While I loved what I could see my fear of being under water (even shallow) was the winner. Going in the Submarine gave me the same visual without the fear of water getting in the way.
I was able to get some wonderful pictures as well - even one of this huge fish up against the porthole where it looks like its giving us a kiss. lol
I am in a similar situation to you - going on a first cruise but not until October so a longer wait; but we were debating the same question as to whether to book before or not. Looks like it might be better to book before but I think I'll wait for awhile.