Normally, pyros underwater can't use their flamethrowers. However, pyros hovering above the surface of the water (by continuously pressing SPACE) can burn submerged enemies right next to them. For some inexplicable reason, Valve has yet to fix this.
We consider this an exploit, and don't allow it on our servers. This rule also extends to pyros burning the bridge from below.
We understand that most newcomers may not know this rule, so we extend the courtesy of a warning when you do it the first time. You were kicked because an admin observed you doing it again (albeit by accident).
Feel free to PM me if you have any questions about this.