We covered books, and some ideas for personal gear. All very practical. But there's more to cruising than pure practicality, so think about those things which will be part of your everyday enjoyment. For us, most of those relate to spending time in the water. With the opportunity to swim in warm, tropical seawater just steps away, there weren't many days we stayed out of the water.
Niall diving in French Polynesia
Snorkel gear: we picked up gear at yard sales and rummage sales over the course of a couple of years. Average price: ridiculously cheap. It wasn't high end gear, and many were lost to leaks, but it met our needs really well for the first 1-2 years of cruising. There was something for everyone, and that meant it wasn't a tragedy if a mask was lost overboard or left on the beach (not that we know any children *cough* who would do such a thing *cough*). We started replacing gear around the 2 year mark, partly because our own standards were increasing and partly because what we had on board was finally wearing out. We're now systematically replacing everyone's gear, and wow, it's pricey- easily over $100 per kid and several hundred per adult. Deals are sweet if you can find them.
Wetsuits. Just because it's the tropics doesn't mean you won't get cold in the water...it's still well below your body temp. A thin wetsuit will help extend your time in the water. We wore them extensively all through the tropics.
Rashguards. These are primarily for sun protection, but also protect you from jellyfish. This isn't idle cruising FUD: poor Mairen was stung by jellies on one of her first swims in Mexico. We were able to get inexpensive, custom rashguards for full body coverage in La Paz, Mexico. Almost every cruiser there can direct you to the lady who makes them in town! The kids loved their "jellysuits" at least in part because they could pick their own material out. The sun rots them, so have backups for when your working set finally falls apart. Union suits or two-piece top/bottom sets are a matter of preference.
Beyond snorkeling: if you dive, this is another obvious area to source gear. We're considering dive gear, and weighing it against getting a hookah rig. Not knowing just how much time we'd spend with our heads underwater when we left to go cruising, we didn't consider the high cost of equipment (not to mention allocating space for the gear on board). It was far to expensive to acquire either of these in Mexico or the Pacific.
Kayak. These are great backup tenders or transportation and entertainment in their own right. I have spent many happy mornings gliding over a lagoon, solo or getting some 1:1 child time... looking at fish and coral and talking about the day. For many reasons, I love our kayak. It was a Craigslist special that lived in the garage before it lived on the bow of Totem. Nothing fancy, but pennies on the dollar of new- especially after leaving the US. Kayak paddles were harder to find, and we ended up paying retail for those in California. As a result, our two paddles cost more than the kayak!
Puget Sound, with our friends from Koi- before we had a proper paddle
Surfboard. Ours was a gift from my aunt, Julie. I love it somewhat irrationally- because of the connection, no doubt. It's something we wouldn't have spent the money on to buy for ourselves before we left, not knowing how much enjoyment it would bring. Many waves (and many afternoons of kid-floating) later, it's hard to imagine being without a surfboard! Now we're looking at adding a stand up paddleboard and possibly a second surfboard (because it's always more fun with a friend!).
Floating in the lagoon, Makemo, French Polynesia
Speargun. We got into spearfishing more than we ever expected, due in great part to two folks: the example and support from our friend Ethan on Eyoni (the guy who speared a fish so big, Latitude 38 thought it was photoshopped!), and the support and advice of our "home island"er Robert, who emailed tips and ideas while we were getting started. If you think you will too, it's another somewhat pricey item that's unlikely to come on your boat and good to shop for bargains or second hand. If you'll buy one online, try to get your hands on one first so you have a feel for a length that works: something you can easily reload in the water, in full gear. Don’t get the type with a screw-on tip. Tips come off too easily, and those spears weren't replaceable in the Pacific (the integrated type was readily available).
As long as we're talking about playing in and around the water, here a few more things that come in very handy.
Lifejackets. We have both inflatable (for comfort in most conditions) and not (for when it's really wet and you don't want an accidental inflation). If your boats comes with PFDs, they will be the bulky Mae West style that helps meet USCG regulations when you have extra bodies on board, but which nobody wants to wear, ever. Children grow and need additional sizes in waiting. If you generally use inflatables, have lots of spare kits for re-arming: label them clearly before stowing so there's no problem matching the right kit to the right PFD later. We could not find these in Mexico, they cannot be shipped, and I'm not sure they can be carried on airplanes either.
Dry bags. When (not if) you flip your dinghy: having your fancy camera safely stowed in a dry bag will suddenly make that $15 investment invaluable. We have a waterproof backpack, but it's not comfortable for any meaningful weight/distance. This may sound crazy, but there were even times we were swimming to shore instead of taking the dinghy. It might have been the heat. It might have been the surf, and not wanting to roll the dinghy. It may simply have stemmed from being stranded because a partner/guest/child has taken the last of the tenders to shore already. Whatever the reason, but you can throw stuff- like a change of clothes!- in a dry bag, swim in, and carry on.
Next in the series: boatliving daily basics.