The thing to watch out for is that in order for the magnets to be able to assuredly pull a boat, they need to be strong. That means you could run into interference if you put them too close to your radio or servos. I'd use a piece of steel attached to the inside of each boat hull and have a towline with a magnet, covered in soft foam. When you need to recover a boat, just leave an inch or so of towline dangling off the rescue boat, tie it down tight and then go swing by the 'casualty' carefully. Small neodymium rare earth magnets should do the trick.