When you came across this case, you should be careful to recover Video on iPhone.
1. Fish your iPhone out of the water as quickly as possible.
2. Dry it off with a towel or something similar. If it is still on, turn it off immediately. Note: running power through wet electronics can damage them beyond repair.
3. Remove the battery, SIM card etc off the phone. If possible, remove the rear glass panel too. Apple uses two special 5-point Pentalobe screws to secure the back.
4. Grab a bowl and fill it with uncooked rice or you can also put your iPhone 5 in with a bunch of silica packets.
5. Bury the iPhone in a bowl of rice and cover it up.
6. Last step is: wait day or two.
The google+ teach you how to recover video from iPhone: https://plus.google.com/106752978783334975190/posts/RL9EhQcLgEx
After this, you may chech your iPhone, if it is damaged, you should recover lost file from iPhone, it may be recover, if you are lucky.