If something is shoalier, it has more shoals or is more like a shoal (a shallow place in a body of water).
Shoalier can make people think of shallow water, maybe at the beach or in a river.
Examples:
The water near the shore is shoalier than the water in the middle.
The fish like to swim in the shoalier parts of the river.
The shoalier it is, the easier it is to find shells.
The shoalier the water, the more fun it is to play in!
The shoalier the water, the more I can see the fish.
The shoalier the water, the more I like to swim in it.