You don't say if you want a permanent setup nor how easy to make you want it. But if you want a permanent arrangement and prepared to do some boat work this is how ours are arranged.
A little inboard from the toerail at the lowest point in the side deck
waterways on each side of the boat we have a deck
drain around 1-1/2inch dia (might be a bit smaller, I am not on the boat, but thereabouts) to drain the deck waterways so's water does not collect against the toerail. In our case, because of the beam of the boat and the fall on the decks this lowest point is slightly lower than the waterway is at the less beamy stern so deck drains or a freeing hole through the toe tail (which is what is normally done) was necessary to stop water collecting and sitting against the toerail when in the marina. But the following will work even if you don't normally need deck drains.
Each drain hole is situated so that its inner highest side (from the athwartships slope on the deck) is lower than the perforations in the toe rail so water goes down it rather than overflow the toe rail. It is piped straight down to below the waterline through a seacock to avoid staining of the topsides but one could just take it out through the topsides under the sheerline as is done on some boats for deck drains.
A short distance below the deck is a Tee arranged so that straight through is vertical. There is a ball valve just under the Tee in the vertical drop to the seacock and another on the horizontal run. The horizontal run goes to a water tank - we have 2 tanks, one connected to the starboard drain and the other to the port drain.
These deck drains are used for normal filling of the tanks and for catching rainwater from the whole of the deck. So, to fill the tanks one just closes the valve under the Tee and opens the one on the horizontal run from the Tee which goes to that filler's tank. If using a hose one just pokes the hose in the deck drain or, if one sees that it is raining one just gives some time for the decks to wash clean of salt
(only takes a minute or two in a downpour) and do the same thing with the valves (without having to go out into the rain and get wet
). After filling the valve under the Tee is opened again and the one in the horizontal run off the Tee to the tank closed.
Instead of two valves and a Tee one could use a Y valve but we figured that using a Tee with the horizontal run to the tanks gives a secure system. Even if any malicious person figured out that the deck drains were used for filling the tanks and poured something down them, it would just go straight out the bottom of the boat through the seacock (which being a deck drain is normally left open). With a Y valve, assuming fitted normal orientation, the contaminant could be caught and held.
As I say, not a temporary arrangement and takes some chopping holes in the boat (we had the builder
do it while the boat was being built) but has worked extremely well for us and I would always do the same again. With the catchment off the deck it is surprising how quickly one can collect 100 gallons even in not so heavy rain. We sometimes have untreated rainwater left in our tanks (they are dark) for months at a time and have never had a problem with it becoming tainted from stagnation.