What size is the lighthouse and where on the deck will it be located ?
A regular sized lighthouse and dock?
Hi Jan: There are a couple of things you can do. 1is to use a waterproof glue like E6000 or LocTite. Another thing you can use is a double ended screw. https://www.homedepot.com/p/Everbilt-5-16-in-18-tpi-x-2-1-2-in-Double-Ended-Coarse-Steel-Dowel-Screws-811341/204274977 You screw one end into your project and then, drill a hole into the dock a little narrower than the screw, and screw the post into the dock by turning it around and around. Here are some more double ended screws: https://www.homedepot.com/s/double%2520ended%2520screws?NCNI-5 Good luck
Hi Jan. You could glue L brackets on the underside of your lighthouse, then screw the brackets into the dock.😊