This question is one that has sat in the back of my head ever since i first played GTA Online for this first time to find out that rockstar didn't give us the ability to have scuba gear, yet in singleplayer it's easily accessible.
This is the one feature i assumed rockstar was going to emphasize on for online play; being able to explore the sea with your friends and find hidden treasures, Easter eggs, underwater bases, and who knows what else! I mean, the possibilities are nearly endless with the amount of things rockstar could do with underwater, and yet they have taken advantage of literally nothing having to do with the water, the only thing that's come close is the beach bum pack which was honestly the most useless DLC in the history of ever.
It's sad to think that there's 3 submarines online that serve literally no purpose except to go 5 mph in a sea of action-less space. It's sad to know that rockstar will more than likely never read this, although I hope that they already understand how much they have missed out on by not including scuba gear online, i mean really, what's preventing them from putting them in online??? There's no technical reason why, it's not like how they restricted the lasso in RDR for online because consoles couldn't handle that many lasso tools in 1 session, scuba gear would cause no coding problem...
One easy way to create purpose in the ocean would be to have "Crate drops" on the sea floor, and who-ever is able to dive to the location first and locate the crate gets a reward that's way better than the normal crates, or even special crate drops. Not only this, but you could have 5-10 friends all deep water dive and explore the sea-floor. There could be missions where you and your friends are assigned to infultrate an under-water base full of enemies, or do a really risky dive in an area that's frequented by hordes of sharks. Rockstar could place countless easter eggs all over the sea, they could have underwater caves, and so much more.
I suppose all we can do is wait and hope that rockstar gets the message. Oh yeah, and we're still waiting for those heists... 0.o