The Rotom Bicycle in Pokémon Sword and Shield is a handy device. You first receive it on the way to the second gym, on Route 5, where you encounter a scientist bullied by a pair of Team Yell members. After the scientist gives it to you out of thanks, you can travel much faster around the map. Thankfully, the Rotom Bicycle comes with an upgrade to go across the water.
To add this upgrade to your device, you need to progress further into the game. There’s no shortcut method of acquiring it or finding a way to speak to a particular person.
The Rotom Bicycle upgrade is given to you by the same scientist, whom you encounter again later on in the game, on Route 9, shortly after you’ve gone through Circhester.
With the upgrade, all you have to do is have your bicycle out while you’re approaching water and continue to ride into it. Your bike deploys two large balloons on your wheels, allowing your character to pedal across the water as if it were land.
Doing so allows you to catch Pokémon casually strolling through the water. It’s similar to all of the Pokémon freely wandering the wilderness in the Wild Area on land. It’s important to note you cannot interact with any of the watering spots while on your Rotom Bicycle to go fishing. You need to hop off your bike to do that.