In the long run, you'll have to make two trips to Warlic's Lake. This first trip is concentrated on the shore areas. There are a wide variety of new enemies here, though most are just tougher versions of those you've already seen. In particular, watch out for Blue Vortexes. Their movement patterns are odd, and they're very difficult to fight from the coast. Once you have the Aqua Boots, it's a much fairer fight.
There isn't much to find in this area without the Ring of Invisibility. For now, just head directly northwest toward an elongated pond. Strike the trunk of the palm tree three times to reveal the wise woman selling the Aqua Boots.
The Aqua Boots open up the map tremendously. You can actually visit any screen in the game right now, even the ones you're not really supposed to. However, you can't accomplish much without the Ring of Invisibility, which is hidden back in Fosbus' Swamp.
The first place you can explore is Warlic's Lake itself. You can't find Warlic just yet, but you can find three Bibles and a Life Potion. If you want to maximize your profit and survivability, pick them up now, but regardless, your next destination is Fosbus' Swamp.
On your way back, you may want to take some time to restore some of the gold you just spent. The Aqua Boots make farming much easier: if there is water nearby, use it to stay away from land-borne creatures while you fight them. There's a particularly good spot to farm Red Jellyfish along the western river.