Jason has super human strength literally folds the sheriff in half, yet a rusted chain and a rock that tommy Jarvis can lift, holds him to the bottom of the lake?!?
Valid point. Back to the rock and chain thing - its tough moving around in water, so maybe that helped keep Jason down there. After being resurrected via lightning, telekinesis, and electrocution, belief of anything in this franchise is suspended haha.
Also keep in mind he was struggling and possibly would've escaped until the propeller from the boat's motor chewed up his head and neck (miraculously leaving the hockey mask in place).