If the almost three year old would be happy with being dropped off in the club and having to "fend for himself" among other kids of various ages with him being the youngest with minimal adult supervision then wait until he can go to the clubs. (Edited to say we cruised the day after DS turned three and he loved the clubs. His sister was in the club, too, though, even though they did not stick together. DS was too busy charming the bigger girls in the club and the cm's. Lol.)
If not or you are unsure, go before and put both kids in the nursery together when you would like a break. Yes, you will pay for him in The nursery, but his cruise fare will be cheaper.
You can take him to the club and play with him there during their open hours. I do not think an almost 3 yr old will be bored in the nursery. They have fun toys and games and a very strict child to cm ratio. It is much quieter and calmer in there.
You must be fully potty trained, no pull ups, to be in the pool on a cruise ship. Quite truthfully, the pools are small and often very crowded. My two water babies (well, they aren't really babies but under 8) have not yet been in the pool after two cruises (4-night and 7-night.) Depending on the ship there is a splash pad for them to play in instead.