Probably seal blubber. Rendered seal oil,was the number one source of tar,
to seal seams and generally waterproof ships.

What materials did you use for the hulls? The skins? I thought about you
the other night while I was watching Nova on PBS. They had a show on
Shackleton, and I thought about you as they were talking about
"seaworthying" the lifeboat for a major ocean crossing. They mixed
something with seal's blood to fill the seams to make it somewhat more
secure. Forgot the other ingredient. Of course you probably wouldn't be
anywhere near the south pole, but I always keep my eyes and ears open for
concepts that might help your efforts/education. Not exactly sure the
seal's blood mixture would be all that much help, however, as the lifeboat
while remaining on the surface was still a leaky watertrap not even
halfway
through their crossing.

Wish you luck with your trimaran! Keep us updated.