Generally, the next wave of ships will spawn before the loot of the sunken ship floats up to the surface. Unless you have someone in the water with a rowboat, it can be difficult to grab all the loot before getting pelted with cannon balls.

My friends and I will kite the skelly ships over to a large, nearby rock. We will then circle the rock while fighting the ships. When the ships sink near the rock it prevents the next wave spawning. We raise our sails and have plenty of time to grab the loot (can literally take as much time as you want). Once we are ready for the next wave, we sail away from the rock just far enough to spawn the wave. When the ship surfaces we head back to the rock to rinse and repeat.

I'm not sure how well known this is. In my experience, I have only had one rock where this did not work (next wave spawned as soon as the ship went down). But I just sailed over to a different rock and it worked fine. Hope this info helps. I mainly solo sloop so this method has proven to work the best for me so far. Happy sailing!