My guess is that the sea troll keeps pressuring Ippo so much he has to attack at a very high pace and his cardio/pulmonary functions can't follow that high output rythm.
If you think about what you've seen, everytime Ippo has been slowing down for any reason, the sea troll placed a very dangerous attack and even connected.
So Ippo reacted by increasing the rythm of his attack, which was the exact purpose of the sea troll.
For the sea troll, this is a gamble, because he still takes strong hits. But my guess is his stamina is enough to sustain enough damage and then attack right when Ippo can't react anymore.
After all, they implied that guy can swim under cold waters at high depths for 13 minutes, without insulation suit. Of course, the way they showed that, you could always say he was gone for 13 minutes of which less than 5 were underwater... but this is animeand to my knowledge, getting such a fish is only possible at high depths and you need time.
So 13 minutes, actively swimming, high depth, cold water no protection. When the world record seems to be 11'30" static and with protection against cold.