Follow the cave further until you reach the beginning of the water- then head down the tunnel to the left instead. This one deepens considerably, which is good because the ceiling drops, and you can swim through into a smaller, round area with air and glowing fungus at the top. Stop here to catch your breath (taking a short dive to the bottom to apply your lockpick to a chest of minor goodies) and then get ready. The next stretch of underwater is long and a bit complicated, and you can easily get disoriented and accidentally drown.
Once you're braced and ready, dive underwater a short distance and proceed down the tunnel you didn't use to reach the room. After a few twists and turns, you'll find the water becoming shallow again, and you can gather yourself before the side-tunnel returns to the main cave.
Advance very carefully from here- while there are some attention-grabbing mushrooms just before you, the cave extends to either side, and a cautious listener can hear the sounds of potential foes in both directions. To the left is only a smaller frostvenom spider, but on the right are two Falmer- one close enough to see clearly, another further off on the opposite side of the tunnel. If you're especially worried about the spider jumping on your back, you can backtrack through the underwater tunnel to come back from the far side of the spider, which is impeding your return to the surface.
Both of the Falmer you can see from here are archers, which will give you a really hard time if you're not at least decent at ranged combat. Lots of jumping is needed to get to their platforms from the water, as there are only a few spots where they descend close enough for walking on- you may have to just cut the pale men off at the ankles.
Take care as you fight, for a third Falmer with sword and shield waits a bit deeper into the cave, and will often be attracted by the sounds of combat. If you're sneaky about the first two, though, you can take him on separately. Where he stands there are several more mushroom patches to gather, and then you can look further into the cave, where some Falmer structures bracket a deeper tunnel.
