How where can I find a submarine I can use at my own leisure?

Yeah it is at the Sonar Collections Dock at the end of the pier, I don't know whether or not you have to buy the dock but I did and it's there. Hope this helps

At the Sonar Docks look for the woman (As Michael) to get the Strangers and Freaks Quest to recover the submarine parts and that will get you the dive boat with scuba gear.

Then buy the Sonar Docks to get the submarine and the side-mission slash collection to get a sub you can use to recover the 30 waste barrels.

At that point the sub will always be there for any of the three characters as will the dive boat.

The submarine can be found after you buy the Sonar Collections Dock and it is at the end of the pier. Also, if you didn't know this, when you get out of the submersible you will have scuba gear so you can swim underwater without having to come back up for air. This does NOT increase your lung capacity. There is also a Zodiac boat that can be found at Elysian Island.

I found a sub as Trevor and it was in the back of the gas station that's is directly in front of Trevor's air strip I bought the air strip but I think you only have to do the mission I never new it was there intell I was looking for a new dirt bike and stumbled across it now I just bought the cargo bob to grab the sub with the latch that the cargo bob has it comes out when you press right and you just hover it right above the sub and then it picks it up

You have to buy the dock property.....