'My Other Yacht Is a Support-Yacht'

Well, of course it is...

 Support yachts are for people who have outgrown their mega-yachts and need more space for their helicopters, speedboats, launches and submarines. The support yacht acts as a floating garage, trailing alongside the main yacht with all of the owner's oversized "toys."
While support yachts have been around for a while, the yacht-maker Amels has launched the Rolls Royce of support yachts -- the Sea Axe -- a gleaming, 220-feet ship that can hold a mega-chopper, loads of speedboats, a submarine, and all the scuba gear, jet skis and floaties an oligarch or sheik could ever want.
Article here 

Never thought of that... we've just been towing our Submarine for years.