Fins should fit snugly; if you can fit multiple fingers in the space between fin and skin they are too big. Fins should not be loose. A loose fin will cause chaffing and likely painful blisters in the long run.

How tight should my fins be?

Your fins should have a snug fit on your feet. Just like hiking shoes, a fit that's too loose will result in uncomfortable chafing and even blisters. At worst, your fins may even slip off while swimming! ... You should struggle to fit more than a couple of fingers into the foot pocket while wearing your fins.

Should you wear socks with flippers?

The main purpose of dive socks is to provide comfort and a snugger fit while wearing full-foot fins. Even with a well-fit fin, you can get excessive chafing and rubbing around your toes and ankles that can cause blisters. ... For hiking boots, that's why a good pair of hiking socks (and maybe some Moleskin!) is essential.