I use the 2 part weatherproof connectors that SCREW together with the wire lead tightened into set screws inside the housings. The bottom portion of the connector is screwed down to the deck
with appropriate sealer used.
Another type for wires that can pass right through that can be disconnected inside the boat at the instrument and then pulled out through the opening are Blue Sea Systems Clam Connectors.
All sizes and styles of both types of these are available at most marine
stores and chandlers, try Defender, West Marine