As you can bring your Pokémon out into the overworld at any time, there is the factor of water. When you are in the water, some Pokémon may not be able to swim. Normally this would just be an aesthetic change, as shown in Pokémon Legends: Arceus where some Pokémon had a platform on which to battle, but in Scarlet & Violet, there is more to it.
With the addition of Auto Battle in the Let's Go mode, the Pokémon can be sent out to defeat wild Pokémon. This is especially useful in Mass Outbreaks to take the Pokémon out to boost shiny chances. However, due to the swimming mechanics, some Pokémon are unable to do this.
If a Pokémon can swim, it can take out all Pokémon in the Auto Battle mode.
If a Pokémon can fly, it can take out Pokémon that are on the water's surface in the Auto Battle Mode
If the Pokémon can only be on an inflatable platform, then although they can follow you, they cannot be used in Auto Battle Mode.
As such, there are some Pokémon you would want to avoid using in the Auto Battle mode when hunting in the waters.