Recently, Aquaman has undergone some changes. He's been re-vamped so that he's really strong on land, as well as being a generally handy sort of guy to have around out of water.  In the past, this wasn't the case. How, then, was he useful to the Justice League? The answer is simple: 
Every once in a while, something would happen where his set of skills were invaluable to saving the world (or someone). 
You have to wonder exactly what the chances are that some evil 'genius' would think building an underwater lair is a good idea if they knew of Aquaman. It's like Gilligan's Island, with that guy who can build a freaking television out of coconuts but couldn't fix the freaking boat. I know, I know, that would have ended the series, I'm not a MORON, I'm simply saying that the writers could maybe have come up with some excuse as to the whole boat thing. My point is, it's ridiculous to believe that anyone who can honestly attach the word 'genius' to his name would fail to take Aquaman into account.  
So my theory is this: any character can be useful in a situation, no matter how stupid their powers are, ergo, if they are deemed popular enough (or if a writer likes them enough), they can, at any time, be given their own series, in which only they can save the world or whatever. 
Let's say someone has the superpower to, I dunno, summon a kitten from another dimension every 2 hours. I can't think of how that would prove useful, but someone out there could, potentially, like him enough to give him his own comic. The costume would be something to see... 
So that's my theory. Remember, you've been warned, it could theoretically happen to any character, at any time.

Aquaman usually was strong on land as well as the sea, it's just that most of the people that write him in stories think he's a joke

" Aquaman usually was strong on land as well as the sea, it's just that most of the people that write him in stories think he's a joke "

" @Emperor Gonzo Noir said:

" Aquaman usually was strong on land as well as the sea, it's just that most of the people that write him in stories think he's a joke "

This, I believe Aquaman jokes came when he was on the Super-Friends and his own abilities were talking to fish and swimming. "