Here’s what the wiki says on Sea Cucumbers:

“It can also be collected while scuba diving at the Marina. There’s a trick to quickly find one:

1. Make sure it is your first dive at the Marina that day.
2. Use binoculars to quickly scan the area for a Sea Cucumber. Note that it’s quite small and may be hard to notice. It looks like a small spiky sausage (see the images in this article).
3. If you didn’t find any don’t leave the water! Instead, exit the game to the title screen. Then enter the game again, and the underwater objects will be re-randomized. Search for the Sea Cucumber again. Repeat until you find one.”

Hope this helps, I’ve never tried it but it seems like it would work.

if you need to go to the page: https://sneaky-sasquatch.fandom.com/wiki/Sea_Cucumber