###No preparation survives the first contact with the players!
No preparation survives the first contact with the players!
This is an important GMing lesson to learn. Otherwise you would be entices to prepare more when you actually should be preparing smarter.
###Preparing smarter
Preparing smarter
You will often here of amazing experienced GMs who can improvise their way through everything. The trick is: They don't improvise, they are prepared. They might not think that they are but all the "experience" they have gathered is actually just unfocused smart preparation. And it is really the secret to good improvisation: Prepare in such a broad and general way that you'll always have something useful for the situation at hand.
For example: There is this cool improv theater piece about a researcher who taught an animal to play a music instrument. The audience says what animal and what instrument and two actors have an improvised conversation about it, an interview. All the while a third actor translates the interview in humorous fake sign language. You will be surprised, how much the troupe can prepare here. Regardless of animal the main questions will remain the same. Few specific questions will arise from certain categories of animals: Swimming, flying, big, small, swarm. Then you just prepare the signs for most known animals and music instruments and you are done. And if they throw at you something, you didn't prepare for? Use the closest fit! They won't know that you are using the prepared material for "bees playing saxophone" to describe wasps playing an oboe.
###Apply this to your GM prep
Apply this to your GM prep
DON'T prepare rigid locations with set characters. DO prepare locations according to their functionality (a seedy bar, a serene temple, a crestfallen warrior, a crazy drunk). DON'T write whole monologues or scene descriptions. DO write some descriptive adjectives and other important keywords or catchphrases. DON'T force the players to follow your story. DO populate the path they have chosen with the things you prepared. DON'T invalidate their choices by looping it back to the plot. DO honor their choices by altering the coloration of the narrative and having in world consequences. DON'T be afraid to prepare content and then not use it. DO write down details of places you visited in game and reuse them when appropriate.
###What to do in your particular situation
What to do in your particular situation
So you prepared an overland journey to a rift and an epic showdown for closing the rift. The players want to go sailing. How to bring those together? Well the land encounters you prepared can still be used. Ocean has islands, hasn't it? Just "reskin" the bandits to pirates or buccaneers and the goblins to native parrotman or something else vaguely similar. Leave the stats as is. Maybe add or change a feat or trade a point of to hit bonus for a point of AC if appropriate. Populate the ship and the island stops along the way with the same archetypes of characters you intended your players to meet on land. Have their choice have consequences to the main rift closing adventure. If the boat route was particularly clever move have the closing of the rift be easier or the land not as ravaged by it, when their done. If the sailing was a detour or if they abandon the rift all together have things get worse in the world. And if the rift had to be closed for the world not to be destroyed have another adventurers do it but at a much higher cost of lives and damage to the surrounding area. And also now those other adventurers have the glory.