On my 1999 H-310, this fitting was factory installed to pass multiple wires down through the deck using one hole :
Although this device presents a torturous path for any spray to follow to get below deck, it's not a good choice where you expect waves to be breaking over the deck. The underside of this thru-deck entry can easily be inspected from below for any leakage and so far, nothing in 26 years.
This pic is quite old and I have since passed two additional wires though the hole. The only other alternative I can see for 100% sealing is individual cable glands for each individual wire.