I actually was forced to *buy* and *use* silicone sealant this year :-(
Unfortunately I needed to reseal the astrodome and if you are sealing acrylic plastic, you CANNOT use any sealent that has any solvents in the formula or it will craze your plastic. Silicone is the official reccomendation Bloody horrible stuff to work with if you want a neat finish, and getting the suface prep good enough so it actually sticks properly is a bitch.
Generally speaking I like a good butyl (rubber) mastic or a polysulfide sealent for above waterline stuff. I used to be able to buy a butyl mastic called Farocalk. Unfortunately the company went under and you can no longer get it. Might have something to do with it having a useable shelf life of about ten years and several years after opening. Fortunately there are alternative brands but they dont sell it in the retail marine marketplace
Most of the modern sealents are only useable for a few months after opening however carefully you re-seal the tube and even if you keep them in a ziploc bag in the fridge with a sachet of silica gel. If you just put the cap back or plug the nozzle, it will set in the tube and you'll be lucky if you find any thats still useable a month later even if you rip the bottom out.
Under the water this year I've been using one of the multitude of Sikaflex products. I probably haven't chosen the best one for the job, but the need to 'git er done' and local avalibility wone out.
Remember they dont call it 'silly-con' sealent for nothing . . . .