Simple non-return valve with rubber flap, suitable for many different fittings including thru-hulls and skin fittings. Simply presses into to provide resistance against back flow.
Available in 40mm and 50mm sizes (external diameter of the white valve ring)
Note that this will not prevent 100% of water from flowing back, particularly if the water is under pressure.
If fitted into a flexible hose / pipe, these can be fitted inside and retained by a hose clamp.