You're not proposing anything new though, and the situation is deteriorating.
Volunteer dive days already happen and guess what? Non-professional divers usually have to pay for their diving anyway, thereby limiting the number of such dives that they do. Also, the same dive sites are usually visited due to being easily accessible and enjoyable for customers.
If you ever visit more difficult, less frequently visited dive sites, I guarantee you'll find a heck of a lot more trash than on frequently dived sites.
I'm a pro and have been involved in marine conservation for several years, and every time we visit less accessible sites it's the same old story. Large ghost nets, miles of fishing line and too much plastic to remove in one dive.
Funding pro and amateur divers so that the cost of their diving is covered, and to incentivise their hard work is a practical way of addressing the issue of ghost fishing gear and plastic debris pollution.