When sailing close to shore, watch out for rocks above and below the surface.
Be wary of shallows near to swamp and plains shorelines as mobs can reach you from the shoreline by swimming, wading or flying out to you.
When sailing on the ocean, watch out for sea serpents.
As for the weather... when it's stormy, trim the sails to level 2 and avoid wave breaking.
Try to avoid sailing at night in unknown waters or when visibility is negligible.
Don't use a raft to sail across ocean ;)