Good question

I thought this too but it isn't a far reach answer: a crab could surface and they could ask it to cut open the plastic. Or they could tear the bag on a rusty nail or anything sharp for that matter.

Well the puffer fish could also pop the bags since their spines are sharp.

Seagull's beak. Mine!

How much would it have sucked if at the end of the movie a person sees them floating in the bags and picks them up "free fish!" and took them home for their tank thinking someone bought them then threw them out...