I have had some hull lay-up and fiberglass experience back in my hands on days. I recall calling those alligators (any rough gnarly thing in FL gets that moniker). Basically what happens is when a hull or other glass part is being made in a mold the gelcoat is sprayed in first. If it is soft or the resin layed in over top of it doesn't harden fast enough it causes the gelcoat to crinkle up leaving the pattern you see on your hull. Subsequent structural laminates are laid in thicker layers and usually build up enough heat to make everything cure. Sometimes these brain like areas will have resin visible in the valleys that has seeped through the gelcoat or from the gelcoat itself.