Although learning how to use a whip would certainly be useful, and Ackster had whips in his inventory, there was a limit to what he could do when he was still learning to use the saber and shield. Ackster also felt that the mermen's whips and whip skills would be as easy to replicate using an ordinary whip.
Besides, Ackster didn't have an arm available. His right arm was still missing.
However, Ackster wasn't discouraged by the reminder of what he lacked or the pointing out of a severe weakness.
The mermen progressing to phase two of this trial and adding whips to the fight only excited Ackster. If nothing had changed, defeating the mermen would have been too easy for it to be a trial left behind by the Sea God.
Of course, there could be more after defeating the mermen, but that would be too boring for something the Sea God had arranged.