1) You can get a "sea chest" from any fish in-game currently of June 4, 2015. I don't know if this will change later on.
2) You cannot fish on a kayak! You receive a sea chest by going to a fish shop and purchasing a "fishing rod" and a "lure" to thenfore fish in different lakes/oceans.
3) The exact spawn rate for "sea chests"? I don't know sorry. I do know it isn't easy and isn't hard to get! 
4) You can get a variety of sellables or obtain the rare melees which include Fish Knife (1/1500), Trident (1/3000) and Pirate Swords (1/5000)
(I hope this helps you Woflie)  |
There's a stock amount for those items??
And can you get sea chest from anywhere and anyfish?