Personally, I would use one of two products:
1. 3M FastCure 5200
2. LifeSeal (Above and Below the Waterline)
Both are available at virtually any marine product supplier. I have used both at each dealership I have worked at and have yet to experience any problems with either one. I have used both to seal screwholes for transducer cables and mounts, speedometer mounts, etc., and have also used both to seal mounting holes for engines.
I would use something formulated for marine applications. In other words, don't just go to Lowe's and buy the first tube of silicone that you see.



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