It's complicated, but ultimately the DM's call
Fitting in a space
Let's back up and go from general to specific here. In combat, a creature cannot move through a hostile creature's space, unless there is a significant size disparity between the two of them.
Nor can a large creature (like a water elemental) fit through a space that is smaller than five feet across.
However, the water elemental has a feature that is specifically designed to overcome both of these situations:
Water Form. The elemental can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.
So, RAI, a water elemental is meant to be uninhibited by the physical restraints on movement that most creatures encounter.
Bigby's Interposing Hand
Bigby's Hand is explicitly not a creature, but it is an object. However, it is an object that "does not fill its space" and which explicitly prevents its target creature from moving through its space. RAW, the Hand should prevent the water elemental from moving through its space.
The role of the DM
In this case, the DM is presented with conflicting rules-as-intended. The spell is designed to prevent creatures from moving through its space, period. But the water elemental has a feature that is designed to permit it to avoid effects that restrict movement. Creating a ruling that integrates these two conflicting rules is the job of the DM.
The simplest ruling is that, RAW, the spell applies to all creatures, while the water elemental's ability applies to creatures and spaces but not specifically to spell-objects made of force, so the spell 'wins'.
A more nuanced ruling is that the water elemental's ability cannot be expected to explicitly describe every interaction with all possible edge cases like the spell, and so the DM has to look at how the spell interacts with the world they are describing. Does the spell create a wall of force, completely blocking off the square? Or does it create a giant hand-shaped force, such that the elemental could actually 'slip through its fingers'? The fact that the hand provides only half cover to the caster (and not total cover), and the fact that the hand exerts forces that mimic the movements of the caster's hand, seem reasonable evidence that the force it projects is shaped like a hand, with 'gaps' between the fingers and around the edges. Thus, it is also reasonable for a DM to rule that the elemental could use its Water Form to move past the Hand.
Deciding between these two rulings is explicitly the job of the DM.
The role of the player
As a player, you are well within your rights to say to your DM, 'RAW the spell prevents all creatures from moving through its square, and nothing in the Water Elemental's Water Form feature overcomes that'. If your DM still decides that the water elemental can slip by your Hand, it is also fair for you to say, 'I did not know that you would rule that way when I recently chose the spell; it now appears much less useful than I thought it was, so I would like to have chosen a different spell (and maybe Conjure Elemental instead!).
Do keep in mind that your DM has the ultimate say in how their world works. It is good that you are self-aware enough to recognize that 'you like everything to go your own way'. Try to keep in mind that D&D is a game of cooperative story-telling. The DM, and the other players, may find it a more compelling story to have your wizard be challenged, and sometimes triumph, and sometimes fail or be disappointed, rather than having everything always go your way. If you approach it from this perspective, you might find it to be a more compelling story as well.
