NFL Draft Prospects -- Hardcore Training ... in LAZY RIVER

"Lazy Rivers" aren't just for inflatable rafts and pina coladas ... a whole bunch of college football stars are using one as a TRAINING GROUND for the NFL Draft.

13 potential NFLers jumped in the water at the Regal Palms Resort in Orlando early this morning to train with legendary conditioning coach Tom Shaw, who's worked with huge stars like Tom Brady and Deion Sanders.

Among the trainees -- Timmy Jernigan (DT) and Lamarcus Joyner (DB) from Florida State, both potential 1st rounders. FSU wide receiver Kenny Shaw was also part of the group.

We're told Tom likes to use the lazy river because it's great resistance training -- and helps the players work on leg and core strength.

Tom's camp tells TMZ Sports ... the resort closes down the pool for the players ... but assures us "if there were kids in the pool, they'd be fine ... the players wouldn't hurt them."