A free mod for Witcher 3, by menschfeind13.

Swords on Boat is a mod that automatically places your weapons on a boat upon mounting it and returns them when you get up.

If you set “Use keys” to on, you will have to stand on a boat and press the “Sheathe weapons” key (C by default) in order to place the weapons and the “Lock target” key (Z by default) to take them back. If you press Z and your currently chosen sign is Aard, Geralt will only get a crossbow.

After placing your weapons, Geralt won’t be able to use them (except for the crossbow while you’re sailing the boat) even if you replace them with the new ones in an inventory menu, so be sure to always take back your weapons before leaving the boat (if you use the keys).

There are settings to adjust the weapons and scabbards positions, and prewritten presets you can choose, just remember to always set “Enable changes” to on.