It's a trade route that has been there for a very long time but has recently been compromised by the hobgoblins (and their friends). As said, the hydra could be a fairly recent addition, as well. I am not sure why the hydra would have any incentive for coming out of the water beyond taking multiple shots at anyone crossing the (slimy-slick) causeway. My group got past because once the hydra had a PC and a horse to eat, and the other PCs were doing damage, I figured the hydra would just let the rest go rather than risk death. It submerged and began to feast on steed with a side of halfling. On their return, the PCs approached more cautiously, took some shots at range, readied actions as they slowly crossed (leap-frogging positions) and gave the hydra a good thrashing when it tried its old trick. It, again, figured there were easier meals and opted not to stick its neck(s) out against something that could fight back so strongly.