The vanilla crawl functions like a less specific case of swimming, meaning that the swimming state is preferred over crawl if possible. This allows vanilla players to crawl into water and immediately start swimming.

In the mod however, players continue to crawl at a reduced speed when contacting water. If the water is flowing, a strong enough current can negate player movement against it.
I also found that the crawling state can be entered directly from swimming state while still in water if the player looks upwards while under a ceiling and releases all controls (Note: in the vanilla game, looking down and releasing all controls maintains the swimming state if there is a ground beneath the player).

Not related, but since being in water while crawling is possible, ascending to a full block ceiling will cause suffocation damage (probably caused by #14?)