You basically have to figure out what you use binoculars for. It is generally accepted that any magnification above 7X - unless it has image stabilization - is too high a magnification to hand hold without some kind physical stabilization - a tripod, or something. On a boat, I don't see any way to do this since the boat moves, you can't be still.
As you increase magnification, your field of view decreases. So as you go above 7X, it becomes harder to use as a "spotter" - or finding something you are looking for. As you drop below 7X, while your field of view increases, the size of the objects gets smaller and the ability to identify what you are looking at gets more difficult. As SG said, 7X seems to be the sweet spot for hand held binoculars.
The other number, you mentioned the 30, then the image stabized ones talk about a 40 and often the 50, that is the size of the objective lens in mm. The objective lens is the big lens away from your eyes. Above you mention a 12X50 and a 8X30. So those two would be a 12x magnification bino with a 50mm objective lens and the second one would be. 8X magnification with a 30mm objective lens.
The smaller the objective lens, typically the smaller and lighter the binoculars get. However, the size of the objective lens really affects the amount of light getting into the binocular. This really becomes noticable at dusk or in low light conditions. The larger the objective lens the better you see as the light goes down. Here the 50mm lens really can give you much better results - with or without image stabilization. There is simply more light to work with being passed into the binocular.
I hope I'm not boring you with too many details, but I thought for anyone that may not understand what the two numbers mean, might help.
Another thing about binoculars not talked about a lot is the two sets of lenses, the side for each eye, must be well aligned. When I am evaluating a set of binoculars, I will put them on a stable platform in some way. I then pick an object with good detail in it and get them focused on the spot. Without moving the binoculars, I'll close one eye and then the other, usually several times, while watching to make sure some specific point in the image I'm looking at does not move at all from one eye to the other. It's amazing how many binoculars fail this test!
dj