If you want to improve your photography, you have to know what's wrong with your photographs. That is, you have to recognize problems when they appear in your pictures, and plan a course of action to avoid that problem in the future. Sometimes problems are chronic, and you have to re-think the way shoot altogether. Sometimes they are transient, due to a difficult or unfamiliar shooting situation, and you just have to remember what was wrong so you won't do it again if that same situation arises again.
Either way, the first step is to learn how to evaluate your photographs. If you're new to photography in general or underwater photography in specific, you may have trouble recognizing flaws just because you haven't seen it done right often enough, and/or you haven't made enough mistakes yourself. It's also easy for the new photographer to become so enraptured with the subject of a picture (say, a dolphin, seal, shark, or manta) that all of your attention is captured by the subject. In this case, it's all too easy to overlook technical problems that prevent the picture from being great.
Another common problem when people try to evaluate their own pictures is that they become emotionally attached to a picture, so they rationalize away what's wrong with it. When you're evaluating your own photographs, be sure to try to detach yourself from the photograph emotionally. Yes, it's your work. But no, just because it's a bad picture doesn't mean you have failed. Failure is when you make the same mistake over and over. Success in learning comes when you correct your technique. That, in turn, will lead to success in results. So don't make excuses for bad pictures. Fix your mistakes, and move on.
How to Evaluate a Photograph
There are few universal rules about what makes a good photograph. Mostly it's a personal thing. But most experienced photographers would agree on the technical requiments for a good photograph. Here are a few items that you'll want to consider every time you look at an underwater photograph.
As you'll notice, I didn't list any corrective measures for any of the problems mentioned above. That's something you'll have to figure out for yourself, although there are plenty of resources on this site to help you do that.
Seek Other Opinions
A second opinion is often quite helpful, especially when you're starting. If you know another underwater photographer (or even a 'dry' photographer), ask him/her if she'd be willing to give you some honest opinions of a couple of your photographs. Don't select your worst pictures, or ones with obvious flaws that you've already detected. Rather, take her some of your favorites, and don't get upset if she finds a ton of flaws in the pictures. Listen closely to what she says, and--later on, when she's not around--decide for yourself if the comments are justified.
Don't get offended if she says "it's too dark," or "why didn't you shoot this portrait instead of landcape?" or "this is nice, but the subject is sort of out of focus." It's entirely too easy to overlook your own mistakes. If you don't agree with the other person's comments, that's fine--but then go find another person to look at your photo. If you think that your exposures are just fine and 5 people out of 5 tell you they're too dark, they're probably too dark.
If you have someone look at your photos, be sure they understand that you want the straight skinny from them. Showing people your pictures and hearing "that's a great picture" all the time doesn't do you any good--and that's exactly what most people are going to tell you. That's exactly what I tell people, unless they specifically state that they want my critical opinion of a picture. And if someone asks you for your critical opinion, do them the courtesy of being honest. Don't be afraid to tell them what you think, even if you think your opinion is not valuable. Telling them "I think you should have put the subject swimming into the frame instead of out of it--but I'm sort of fuzzy on composition in general" is much much more useful to them than "nice picture."
The root of all learning is practice. In order to improve your evaluation skills, you need to employ them regularly. It is often instructive to look back at some of your older work and re-evaluate it. You'll be amazed at how many flaws you find in work you previously thought was "pretty good."
Where to Go From Here
In order to help get you started, I have assembled an Evaluation Practice Gallery, which is a collection of photographs along with short evaluations of them from me. You can look at each picture without seeing my comments, and then bring up the comments by clicking on a button. The intent is for you to evaluate each picture yourself, then bring up my comments so you can compare your thoughts to mine. Agreement with me is not the goal, but if you find yourself saying "nice picture" and my comments point out a bunch of flaws, then perhaps you need to slow down and look closer. Also, if you don't understand my comments, then perhaps some more reading on the topic of the comment would be a good idea.