I've run BB 2x now, and both parties immediately figured out "I'm gonna go run in the water since I'm on fire and dunk myself", only 1 party was super upset that it took an action to drop prone. That group was full of 5e "fan-boys" that were all like "I should be able to drop prone as part of my stride action". Once they figured out that the water was safe, they just basically killed the rat from a distance using ranged attacks and it was an easy fight for both groups. But then again I did have the rat stop at the waters edge, then it would hide out of LOS behind the pillar, so they had to adjust, and move in and out of the water. It was a fun and interesting fight for both groups, learning how to move around in combat, and corner the enemy in a way that they had an advantage but could avoid the super nasty effects of the enemy.