Grand Caribe and it's dock are sort of at a point and the boat traffic passes very close to it's dock. I have warned many swimmers of the dangers of swimming out any further than the docks. You are taking your life in your hands by doing so because you can not be easily seen by the boats. Signs where posted a few years ago, after a tragic accident just south of town, warning against swimming out past the docks. It is extremely dangerous and most resorts discourage it.
Many resorts including Grand Caribe put floats buoyed off front of their docks to keep the fast traffic a little ways off. I think they are still there. But swimming between the old Palapa Bar and Grande Caribe is not a good idea. You have private boats, water taxis and commercial boat traffic using these waters so you must exercise some caution and commonsense. I've witnessed at least 6 close calls myself over the years.
In my opinion the boat captain that approached you was trying to do you a favor and prevent you from being hurt.
Somewhere on a beach in Belize