When you are in a boat with kids, you need as many things as possible to entertain. Preferably as small as possible. One that we took to the BVI last year was a GoPro camera. It’s robust, waterproof and you can wear it on your head!
Here is a video I took at The Baths in the BVI. It’s great for just messing around with and now the boys love wearing it and doing stuff like snorkeling, swimming, skiing etc.
One downside is that you do need a computer to easily mess around with the videos, in this case to upload them to youtube. I plan on having a Macbook Air with me, so its pretty easy to take the SD card, put the video on the laptop and then edit before uploading.