A limited number of types of plastics like those used in Beckson Portlights are not compatible with Polysulfide (LifeCaulk, 3M 101) or Polyurethane (5200, Sikkens, etc) sealants. If there isn't a warning from the manufacturer against using the Poly sealants, they are probably okay. Still not a bad idea to check with the manufacturer for something as potentially embarassing as a thru hull. I've used LifeCaulk below the waterline and practically everywhere else for decades with never a problem. Have sealed bronze, stainless, plastic transducer, Marelon through hulls without an issue. The manufactures wisely realize that Sillycone sucks as a sealant and don't use the type of plastic that requires it for the below the water line fittings.

Sillycone belongs in boobs, not boats.