Once you have the SHIP, you can travel across the inner sea to ElfLand, the Dwarf Cave, or back to Coneria. You will only be able to dock at white brick ports like the one outside Pravoka. SHIP travel is fast, but you will be attacked by various Aquatic creatures as you sail around. Even when you gain access to the rest of the world's Oceans, you will always find the same monsters while sailing.
You'll find that most Ocean creatures have a very high Evade, which makes battles against them difficult. The level 3 spell LIT2, sold in ElfLand, can make short work of most of the creatures you'll fight. Another effective spell is DARK, which will reduce the Evade of any monster it hits by 20% in addition to reducing their Hit Rate by a like amount. Spells are useful on the open seas because you will typically dock near a town with an inn, allowing you to restore any used spell slots.
The one exception to the rule is the KYZOKU, a non-Aquatic creature with a normal Evade and a good bit of loot—120 G per kill! These are the guys you want to see, even though they don't have the same vulnerability to Lightning as the other Ocean creatures.
Of the standard creatures, SAHAGS and their more powerful R.SAHAG cousins are the most common and the least interesting. They are annoying to fight, but not nearly as annoying as the OddEYES that often accompany them. These creatures have only 10 HP and can't deal any damage, but they will attempt to paralyze one warrior each turn with their GAZE attack. Take them out quickly to avoid this.
The most dangerous creature on the high seas is the SHARK. They dish out plenty of damage, and combine high Evade with a lot of HP. If you don't have LIT2, LIT is highly recommended when fighting SHARKS. They can appear in pairs or with SAHAGS or R.SAHAGS, making them that much more dangerous.
If you're looking to make money, you're better off fighting on the Ocean than near ElfLand. You'll sacrifice a bit of experience for that extra G (due mostly to KYZOKU), but the fighting is also a bit safer, especially if you have LIT2. Best of all, there is nothing Poisonous to be found on the Ocean.