That sand patch is very short (100 yards or so) - annoying but not that bad.
You can probably go around it if you prefer - take Avenue U and turn on Flatbush (I never tried it)

Rockaway Point Blvd has a nice (shared) bike path.
Open Google Map and select bicycling - it will show you paths in the area - I find it very useful

Marine bridge is a pain to ride - it is narrow and you are supposed to unmount and walk. Noone does of course but you can get a ticket if caught