If yours are the same portlights as I have (circa 1989), the cap in the lock handle is removable and with care and a sharp tool it can be done without damage.
If you can get the plugs out, the screw needs just a small amount of tightening to make a real difference to the seal on the rubber. Also check whether the "glass" (parenthesised because they're not glass) is distorted. I have seen some that look very wavy. If yours are then there's no alternative to replacement.
I have replaced some glass, some rubbers but mostly the solution above has worked (I have 12 of these little buggers on my boat)
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