All of those can be re-flavored/used
Fire spells could use a form of boiling water that is blasted towards wherever the caster chooses. Much like the pistol shrimp does. This is a fairly easy one that can be used in any 2D or 3D water environment, and allows the use of any fire spells.
For Earth spells - The ocean is full of rock constructs. There are underwater mountain ranges, cliffs, trenches, caves, etc. The entire ocean floor or any natural wall would be made of some form of rock that could be used. Unless they are solely in open water with nothing else around, this could probably be used. These actually could be pretty cool in altering 3D environments into funneling the fight to a more beneficial area for the party (say you alter a rock structure that forces the enemy to move towards the plant life that allows other spells like below)
Entangle, or plant spells like it, could be used on any various underwater plant life. Seaweed itself already entangles people enough, magically causing some to do so would not seem out of the ordinary at all. And as shown in any oceanic nature documentaries, there are various forms of floating plant life on the oceans surface, floors, and within oceanic currents. Of course this one would seem oddly coincidental that plant life happened to be nearby when needed. Although it is magic, you can just imagine the seaweed is conjured to do it's thing, it can still tangle you up without being rooted in something solid.
Wind based spells could be flavored as an oceanic current shift that just affects the field as wind normally would. However using spells like Wind Wall would allow for enemies to travel vertically around it in a 3D environment.
Similarly to Wind Wall, spells like Tidal Wave can still use current, or maybe oceanic pressure. A tidal wave does bludgeoning damage, maybe a wall of denser water from the depths is conjured that compresses the enemy into the bludgeoning damage.
The only damage type that I would be hesitant of is lightning damage. As we all know, water is a conduit, and blasting lightning at enemies surely would cause issues to everyone/thing around you. For the sake of allowing the spells to be used, this could be overlooked taking out some of the realism of the world, but that would seem pretty corny. The only reflavoring on this could be a lightning LIKE spell is used that still does lightning damage. Although since the rest of the spell types seem to have a good route to be usable underwater, it should be acceptable that some won't be. I doubt your druid is going to have a spell of each element and might choose to focus more on ones that work normally in this case.
I think the biggest thing you as a DM need to do is to create an environment verbally/visually that allows for these things. When creating a battle map, make sure there are rock outcroppings and plant life somewhere that your druid could use strategically in a battle. If you are in the open ocean and the druid is looking for anything in the environment they could use, throw them a bone once in a while. It's not a ton of extra work to add these environmental factors, and would allow your druid to pick any spell type he/she would like.
