Originally Posted by sailingdog
By saying that you should be potting the holes in epoxy, doesn't mean that you shouldn't be using a good sealant for the installation. The use of the epoxy isn't to provide a water-tight seal for the installation, but to protect the core from water intrusion and prevent a much more problematic repair later on. Boatlife or 3M 4200 are probably good choices, but it really depends on what the speaker and grill are made of. Some sealants and plastics don't play nice with each other.
It's a long shot, but you have to consider that speakers vibrate. It's probably best to do the job thoroughly with epoxy "donuts". If water is going to rain on it , or smack it via a wave, it's just good insurance. If the deck hardware takes a load, like a winch, turning block or a cleat, it's essential.