Can you give us the enviornmental conditions that led to your Rio M "hissing" at the centerboard trunk
hold down screw (s).
Was the water quite cold and the air on the beach quite warm?
The CB trunk cover screws are supposed to be set in inserts the same as the footstrap screws.
If one or more of the screws has pierced the insert, then water may be getting inside to the foam
core of your board. This is obviously not a good thing.
I think you have a good idea with putting sealer on the screws, but I would not use silicone.
If you are in the USA, see if you can find some Hylomar PL32 M. It will provide a permanent seal on any
leaking screws. I would suggest taking a toothpick and working a little Hylomar down into the holes (all of them) before installing the screws again, the idea being to have the screw force some Hylomar PL33 M
to the bottom of the insert (where it's pierced) to seal that up permanently.
I would also recommend that you remove the vent plug and take a check if there is any water showing at the bottom of the vent plug fitting. If it's dry, replace the vent plug.
If not, use a twisted piece of paper towel as a wick to remove the water from the interior of your board.
The way to do this is to use the rise and fall of ambient temperature.
Close the vent during the nite when it's cool (this will pull the moisture out of the core and nearer to the
vent hole) and then open the vent an insert the paper towel "wick" before the daytime temperatures get real warm the next morning (Say before 9:00 AM). Leave your board in a warm sunny spot to allow the
heat of the day to bring the internal temperature up and drive the water in the core foam up to your wick.
Do this as many times as it takes for the wick to stay dry.
Yes, sailors have taped over or even filled in (with expanding foam) the entire centerboard trunk, but
the real solution is to trade up to a shorter lighter board, or retire your Rio M to being a trainer board for your friends and something you use on light wind days when you cannot plane on something smaller.
The stock 410 Shallow Water fin is OK, but as soon as you want to get planing earlier, and no longer need the centerboard/daggerboard to stay upwind, upgrading to a 48-56 cm vertical pointer fin will significantly improve the upwind and early planing performance of the Rio M.
If you have weeds to deal with, or sail in an area with lots of shallow sand bars, then a 39-48 cm weed fin
would be the optimum fin to have.
The EVA deck covering will fade over time, but there is no "treatment" that I know of that will prevent this.
I have Starts that are now >10 years old, and other than fading of the blue color, the EVA deck is still doing well.
A bigger problem, on the Rio line of boards is having small "chips" in the EVA (on the rails especially).
You can use a contact cement (try to find somehing that's clear) to glue the chip back in place.
Hope this helps,