Yes its entirely possible. The first set of trash is attached to Hydross, if you clear all that out, then its possible to bypass Hydross by taking a hard left at the end of the first set of platforms and going right around the edge of Hydross's area. You might pull a few low-HP adds, but you won't pull the boss.
You're then on the Lurker Below trash and have access to the rest of the instance. If you kill Lurker (not strictly necessary, but easy enough to do) then the fish in the water for the whole instance die, which makes life a lot easier.