[Fix] Boats should never get FixZ'd (#2246)

* Boats should never get FixZ'd

* Use member variable to avoid repetitive checks

* Resolve comments from review

* Fix return type

zone/waypoints.cpp

@@ -809,6 +809,10 @@ void Mob::FixZ(int32 z_find_offset /*= 5*/, bool fix_client_z /*= false*/) {
 		return;
 	}
 
+	if (GetIsBoat()) {
+		return;
+	}
+
 	if (flymode == GravityBehavior::Flying) {
 		return;
 	}