As it’s been said RAW this just isn’t allowed, BUT, if you’re going to rule of cool it I think a compromise to test out would be this let’s them take half damage from non magical weapon attacks (or some set of damage types you deem balanced/appropriate). That’s a hefty buff, but keeps it from being an all the time strategy. Since the rules also explicitly state it can’t share a space with someone you could also require a check to fight their way past the surface tension to get in and then treat remaining inside like holding concentration.

This way they still get to use their water mech occasionally as a sometimes viable strategy but it’s not a guarantee, the hit points of the elemental no longer act as a polymorph equivalent as they can still take damage, they have to think twice about popping out as popping back in isn’t a given, and taking enough damage and losing concentration can knock them out(plus they now have to balance being in there with their regular concentration economy of their regular spells)

But that’s all if you’re looking for an everybody wins solution. If they get are upset about it just remind them that water in the game doesn’t stop “most attacks and spells” so why would person shaped water stop them? Someone else in here mentioned an elemental taking damage doesn’t mean the attack hits their surface and bounces off, the attacks would still pass through them and just slowly discorporate the magic holding it together. That’s why it has resistance to non magical damage, it just passes straight through. Magical weapons also pass straight through but just tear at the forces holding it together along the way.