I'm not sure how you're supposed to take on the sea monster but it was balls hard, spent easily 10k dust on mercenary units, kitted my fire boat with mithrite weapons. The invulnerability against endless flame (I assume intentional) doesn't help...

Here's my reward xD.

Not sure this monstruosity should be tuned down or reward tuned up (was looking forward to an artefact or some goodies). But beside achievement hunting and removing this nuisance from your ocean... the reward didn't exactly overwhelm me especially with my income at that stage of the game xD.