Silicone sealant -- $4 for a name brand BIG tube at walmart, $9 for a small tube in the marine section, still at walmart, still silicone.
And if you are permanently on freshwater and your stereo is inside, it is questionable if you really need a marine version. It will probably be long outdated before it actually fails.
Rotella oil for your engine is much cheaper at Advance/Auto Zone than your average marine store.
NEVER buy those nice looking colored lines at Home Depot and try to use them as sheets, halyards, or anything on your boat. They only have a foam core, and will instantly stretch when put under even a light load. (And yes, I found this one out by experience!) I don't even know any application an 'unloadable' line would be good for, other than mean pranks on your sailing buddies!
Regular polyeuethene varnish works just fine inside and holds up well. Don't skimp on the outside wood, I recommend Cetol.
I personally think a shot of vegetable oil works fine to keep the head lubed and working smoothly, as opposed to $8 "head lube." And on that topic, environmentally safe RV holding tank deodorizer and TP seem to work fine too.
And I swear by ebay for most purchases of 'real' marine items.