You can visit this area without the Aqua Boots, but you won't be able to fight Fosbus or earn the major prize: the Ring of Invisibility. Once you can walk on water, return here to clear the area. I recommend taking on Fosbus first, since the Ring of Invisibility is out of the way and won't help you with that part of your quest. However, if you're impatient to go exploring and abusing the Aqua Boots, feel free to get it right away.
The only new enemy you'll find here is the White Bat, but you will find them everywhere and they are quite dangerous. Before fighting the boss, make sure to pick up the two Bibles and the Life Potion in the area. The Bible in the southeast corner is the easiest to find—just hit the conspicuous bush a few times. The second Bible is found in the middle of the western part of the swamp. Head northwest from there to find a Life Potion. You'll have to enter this screen from the northwest to reach the cave. Use the full healing to your advantage to prepare you for Fosbus.
Fosbus' Cave
To find the cave, strike the bush on the east side of the central lake. This is another side-scrolling dungeon. You'll find a number of new enemies here, including Spiders and Fleas, but nothing that is much of a threat. The real threat is being trapped by the dungeon's puzzles.
There are many destructible blocks and triggered platforms in Fosbus' Cave. Generally, if you reach a dead end, your best bet is to start swinging at every wall and obstacle. The first real threat comes from a Giant Snake that fires trios of Fire Spirits. Getting hit full on, especially in the air, can drive you back and force you out of the dungeon, so take care. Shortly after that you'll reach an impossible jump—strike what appears to be a Jewish headstone to lower a platform and continue on.
Shortly after a winding passage full of spiders, you'll encounter the toughest obstacle yet. There are a series of raised platforms, and if you fall off, you'll have no choice but to exit the dungeon. The last jump in particular is a bit tricky, and a Red Snake on the floor may tempt you to go kill it.
The second Giant Snake is an interesting one. It will create a Fire Spirit that will rotate around it like a shield, then fire more off at high speeds. There isn't much chance of being knocked out, and there's more to this area than it seems. The blocks on the floor are all destructible. This is not only your path onward, but it can give you a nice advantage against the Giant Snake to clear a path to him. There is a third Giant Snake, but this one only tosses Red Snakes and isn't much of a threat unless you scroll the screen so far right that you can be knocked out of the dungeon.
You'll find Fosbus after the path branches into three (any path will do). Fosbus takes a lot of hits, and can fill the screen with projectiles, but she only moves horizontally so you can stay out of her dangerous reach. Try to get your hits in whenever she comes to a stop. If you have enough health, you can survive a few hits from the projectiles while you hack away. As bosses go, Fosbus isn't that tough.
Unlike with previous bosses, defeating Fosbus will not open up any new paths on the map. It will open up the cave containing the Crystal just north of the dungeon entrance, as well as caves to the west and south. You won't find useful hints in either—these caves likely exist just to confuse your pre-boss attempts to find caves.
The Ring of Invisibility
With Fosbus defeated, it's time to spend some of that hard-earned cash to buy one of the more important items in the game, the Ring of Invisibility. Despite its name, it's not a defensive item. Rather, it (inexplicably) allows you to destroy gray rocks with three sword strikes. The manual is no help in explaining the name, either—it refers to the ring as the "Ring of Invincibility." The best theory I've heard is that it makes the rocks 'invisible,' which does at least seem to fit with the rock destroying animation.
At any rate, you will have more than enough gold from Fosbus to buy both the fourth Crystal and the Ring of Invisibility. You'll need to traverse the swampy maze to find it. Start off by heading north and then following the upper wall west. You don't need to go all the way around, and can in fact cut your way by heading southwest from Enny. South of there, you'll find an island with a brick floor. Defeat two Red Crows to find the Ring of Invisibility for the fair price of 10,000 gold.
With your prize in hand, it's time to move on. Before heading to Warlic's Lake, return to the northern tip of the Central Forest and buy Aresta's Pendant for another 10,000 gold. This defensive item will reduce the damage you take by 1/4, making your journey much easier going forward.
This also might be a good time to return to Rolick Desert and check out the river near the path from Despa Valley. You'll find a Life Potion selling for a dirt-cheap 500 gold, as well as Purple Mea sold for a game-low 5,000 gold. It's an inconvenient trip, but having Purple Mea available so near the continue point is quite a convenience.