Sea mode gunnery is technically the same as AA mode (the gun can move freely). The only difference is that in sea mode the gun will traverse extremely slowly to allow for precise adjustments in aim. You also get a little range display. Correct ranging and lead is still done manually.
Grizzly's method is a bit of a hackneyed way of using the guns ship-to-ship.
It requires constant flipping back to the clipboard and doesn't teach you proper manual technique.
I think sea mode is preferable in most situations, but it does take a good deal of practice. You need to learn to "bracket" the enemy ships. That is, with your first few shots you should have identified a bracket of possible ranges for the enemy ships, that should get smaller and smaller with each additional shot until you start hitting. For example, watch for the splashes of your shots. If your first shot falls behind the enemy, shorten your range up considerably and fire again. Your next shot should fall short (or hit, in a perfect world), so that now you have two ranges, and you know the enemy lies between them. Keep repeating this, closing the gap between those two ranges, and eventually you will pinpoint the enemy and start landing hits.
It's not that easy, because you have to constantly estimate the relative velocity of the two ships and mentally update your ranges as time passes, and keep leading properly. You get better at this as you practice, and you get better at eyeballing ranges. Eventually you'll be landing shells within a few hundred yards on your first shot, if you keep at it enough.
Also, make sure to use the zoom key. When manning large caliber guns against distant ships, you should always be zoomed as far in as you possibly can while still keeping the enemy ships in your FOV, so that you can clearly see where your shells are landing.
For engaging shore batteries, you should use land mode, as they are static targets.
