I bought one of these gators and they appear to be made of some kind of Styrofoam. They are painted w/ something,but don't know what so there must be something safe out there to seal stuff to make it fish safe.
Just looked this gator product up and says foam molded:
Pond or Pool Decoration/Decoy
LOOKS REAL!!
Scare the neighbors & pests with this lively floating alligator pool/pond
decoration/decoy. Original in style and realism. Durable, attractive, foam
molded. Very realistically painted. Reflective red eyes. Great for swimming
pools and ponds. Size 27 1/2 inches long by 9 inches wide. Manufactured entirely
in the U.S.A.
So I think that great stuff foam idea might be a great idea to add underneath the log to keep it buoyant. Do they make it in a dark color?