Great pictures, thank you!
This looks and sounds like a typical hoof wall bruise – nothing to really worry about, but definitely something to prevent. While soaking will not hinder healing it will not help either. Give Splash as many days off as he needs to heal and he should be good to go.
From what I can see in the pictures his heels are a bit long. The longer the heels are, the more pressure is put on the front part of the hoof and the horse is pushed off balance. Long heels also make it much more difficult for the horse to land heel first (as he should). Also, since his heels are long the hoof wall has to take more of the shock than it is capable of, without much help from the natural shock absorbing system in the heels.
Another thing to consider is that a horse that is shod is much more likely to get a bruise in his hoof wall due to the fact that horse shoes conduct the vibrations of the impact of each step (and jump). On Sunday when he went over a jump he probably landed just a little off balance with more pressure on a particular spot on this hoof. This would cause a bruise in that part of his hoof wall. Bruises can take anywhere from a few hours to a few days to show up and sometimes (as in your case) can make the horse sore.
To keep this from happening again I would focus on lowering his heels. Keep an eye on the rest of his hoof too – especially in regard to not letting the hoof wall get too long, imbalances within each hoof, and pressure points. Just keep in mind having well balanced feet makes a huge difference. If his feet are in optimal condition this will not be a problem for him in the future!
This site also has a lot of information on all types of hoof bruises. http://www.ironfreehoof.com/hoof-bruises.html
Hope this helps!