Check wiki for base structures to see all the different structures that block spawns. While workbench is easiest perhaps to use, wooden torches are cheapest and easiest to hide/protect.

For harbor areas, consider making a lighthouse using an earthen pillar topped with a campfire and at least one 2x2 piece as a reflector (renders from further away). Not only does this make it easier to find the port in bad visual conditions, but I use them to mark the deconstruction area. Side effect is deterring at least some swimming mob types from approaching that area of the port.

I believe torches may also help in deterring anything with a fear of fire so that might help a bit with port security against lesser troublemakers.

[edit] Also earthen 'breakwaters' (tho perpendicular to shore) help against swimmers, particularly if they have a small perpendicular aspect to lengthen mob pathing.