My girls were passionate about making cards for their swimming teachers, so I found 2 zip-lock lunch-bags, cut some watery-blue cardboard to fit and let them collage the cards. They used some mermaid printpapers, ‘bubble’ stickers, blue and green colouring in, and pieces of blue paper to complete the cards, and then on the back they told me the message they wanted to write.
(You can of course substitute all of the above for whatever colour, designs, papers, and decals you wish to suit the thank-you theme you're aiming for).
When they were finished, we popped the cards into the bags, sealed them shut, and off we went. The teachers laughed when the girls handed them the cards while they were still in the pool; no worries about the splishing and splashing, and the teachers didn’t have to jump out to put the cards in their bags, they simply put them on the side of the pool and collected them later.
Maybe the teachers would have preferred water-proof-chocolate better, but then they’d just have to swim more laps to work it off... right?!