If you are going sea fishing but have only gone fishing in rivers before, you will probably need different equipment. Of course a saltwater fishing rod and reel are the obvious pieces of equipment or tackle that you will need but there are other differences you will have to make in your tackle box also.
Rods and reels for saltwater fishing are usually stronger than ones used for river fishing as you will probably want to catch larger fish in the sea than you would be expecting to catch in any river. The bait and lures could also be different. Bait is the term usually applied to live, small fish but can also mean meat cut into pieces whilst lures usually mean artificial items which fish will hopefully mistake for food. Different types of fish are known to be attracted to different types of bait or lure and so the ones you use may mainly depend on what you are hoping to catch.
Just as with river fishing though, a full and complete tackle box is also required when sea fishing. The tackle box should contain, among other things, spare reels, additional hooks, lures if you are not using a live bait box and a first aid kit in case of accidents which may cause cuts. Usually all the items that you will need in a typical tackle box can be bought all at the same store, either local or online and the stores will often have supplies for both fresh and sea water fishing. Although many fishermen carry a small knife in their tackle box in order to cut a line if necessary, some prefer to carry nail clippers for the same purpose which avoids you having to use your gutting knife for that purpose.
Although you should already be using a fishing line which is capable of catching the size of fish you are hoping to catch, the lines weaken after a certain amount of use and so, if you have had a particularly hard struggle with one fish, regardless of whether or not you caught it, you may want to consider changing your line before making another attempt. Unless you are fishing from a pier or from the shore, you should always wear a life jacket when fishing from a boat as sometimes a large fish may pull you over board. It is for that reason that boats properly equipped for seawater fishing will usually have straps to strap you to a seat which struggling to bring in a fish.
Just as with river fishing, different methods are used to catch different types of sea fish and so some knowledge of those is advisable, unless of course you are accompanied by an experienced salt water fisherman. Almost as important as knowing how to fish for certain fish is knowing where to fish for those particular types of fish but any fisher boat captain should be able to place you in the right spot for the type of fish you are hoping to catch.