Skylight filter for APO77 (1 Viewer)

:h?: Does anyone have any experience using a skylight filter on the front of a Leica APO77 or similar? As I have been sea watching this 'summer' I have noticed a build up of spray and dust on the objective lens, more so than usual.
A filter like a skylight 1B will keep the costly glass clean but it must have some affect on the image. Trying one in shop conditions will not show me much.

Hi Quark5, there's plenty of threads on birdforum about filters for scopes, the APO77 in particular, just scroll down to the bottom of the page after reading this and you will see a list of related threads.
Regards,
Andy