My story has a character who is exceptionally skilled in fighting in an underwater environment;

He's a shark.

he has extensively trained underwater and has the ability to control large bodies of water.

He's a Magical Shark.

He is incredibly tall and muscled.

He's a Magical Big White Shark.

He is strong enough to swing [...] melee weapons despite the water-resistance and his signature strategy when fighting is to create a room-sized body of water around his enemies so he has the advantage due to his training.

Yes, even a Magical Big White Sharks has dagger-like teeth. They are perfect for shredding any body when under water.

However I am not sure what the design for his personal weapon should be that could be effective in both on land and underwater combat.

Dagger-like teeth obviously, because your character is a Magical Big White Shark that possibly turns into a human just because that makes it much easier to get so close to the one prey that is available in overabundance: Humans. Oh, and they also grab their targets, then thrash with whatever limb they grab in the mouth, ripping it off and leaving a quickly bleeding out wounded body.

My story has a character who is exceptionally skilled in fighting in an underwater environment;

It's a shark.

he has extensively trained underwater and has the ability to control large bodies of water.

It's a Magical Shark.

He is incredibly tall and muscled.

It's a Magical Great White Shark.

He is strong enough to swing [...] melee weapons despite the water-resistance and his signature strategy when fighting is to create a room-sized body of water around his enemies so he has the advantage due to his training.

Yes, even a Magical Great White Shark (MGWS) has dagger-like teeth. They are perfect for shredding any body to mouth-sized bits when underwater.

However I am not sure what the design for his personal weapon should be that could be effective in both on land and underwater combat.

Dagger-like teeth obviously, because the character is a Magical Great White Shark. That possibly turns into (appears as?) a human just because that makes it much easier to get so close to the one prey that is available in overabundance: Humans.

How do they fight? Well, they charge in on their target with their superior strength and shape while being submerged. Then they open the hatch without lowering their water-resistance too much because excess water is piped out of the gills. Suddenly, the jaws moving at something like 56 km/h bite close around a limb of their targets and lock shut, trapping it. With the huge mass and the superior fins to allow such, the MGWS then thrashes with whatever limb they grabbed in the mouth, shredding it to the bones and likely ripping it out of the socket, and leaving a quickly bleeding out wounded body behind.

Though, if MGWS is large enough, they just chomp on the target twice to kill it, then swallow it whole.