@SPR I have a few OVEROARD duffels, have used them for SIB but not for snorkel equipment.
I think that for fins, neoprene jackets, mask you need mesh bag to dry them ...
The same for anchor. It is not so good if you keep wet rope in waterproof bag.
We use waterproof duffels for spare T-shitrs, normal shoes (we use neoprene shoes for swimming and walking near beach in Greece).
For towels you need mesh bag also ... We are 5 persons family (2+3) so we have a few bags every time
Sorry - you probably suggest it for rescue flares, spare parts for engine ... others