Why the lack of underwater content?

Could you elaborate a bit, how a fight underwater would work ?

Or do you just mean a normal oxygene athmosphere that was placed under water, but the chars wouldn't get wet while fighting?

Using a flame thrower skill underwater surely got an impressive effect

I guess, the whole combat would be needed to be totally revamped. Almost every animation would need to be changed, most attack skills would need modifications or new "underwater" rules.

And to what gain? What is the positive effect that makes all this work worthwhile, when it could also be put into (much needed) content updates of traditional nature?

"Much needed" content of a traditional nature? I hope you don't mean MORE Flashpoints and Operations...do you?
But to answer your question, the benefit would be that it would bring diversity to the game. And that's what's really "much needed" in SWTOR.

While I'd like to see Manaan very very much I wouldn't call what KOTOR did underwater content.

It had a bit on an underwater station and a short walk in a pressure suit that was about as annoying as KOTOR II paragus spacewalk bit.

Personally I'm not really missing the water thing the way mmos usually do it.

Tho granted, it should be in on say Ord Mantel. If you're gonna do water planets having exhaustion zone above your ankles is laaaaaaaame. If they weren't going to do a lame swimming bit then they shouldn't have designed planets with islands and lakes and whatnot. This half tooshied approach just contributes to the feeling that TOR doesn't have a game world in anything but the shallowest stage prop scenery sense.

Take 10 players and you get 11 definitions of content.
I mean more stuff to do in general without overly repeating stuff that you were doing for a while already. This does include, but is not limited to new FPs and OPs.

And could you now please answer my question as this is otherwise going off-topic and the OP doesn't like their threads derailed.

How would an underwater fight work?

Ord Mantell was designed for some kind of water-based system which would have probably included (and, here's hoping...WILL include) both swimming and hydro-speeders components.

Who says there has to be fights? It could involve some kind of pseudo-platforming...like in the old Super Mario games, where your first and foremost objective was to avoid touching the fishes, not necessarily fight them. There might also be the puzzles involved, or time-based quests in which the player has to run against the clock (before his oxygen runs out)
But if fighting absolutely needs to be done, there are aquatic lightsabers the Nautolans use, I suppose—thought it wouldn't make much sense to introduce them in early levels like Ord Mantell (one of the few examples of planets in SWTOR that was clearly designed with water-based systems in mind)...

Except for anything the devs have said...it's always been "we have no current plans for water content" yada yada, you're reading too far into things. It's called backstory, setting, etc. One of my toons has a giant cult...yet I don't get nor are there any hints at economics / political gameplay. But from your reasonings, there is.

uh oh... better brace yourself.

While I personally come from a roleplayer perspective and am perfectly okay with not killing anything for a week (except wookiee. I need at least one dead wookiee per day), I heard a lot of complaining both in game and on these forums about quests involving super-marioesque style.
I have done the bino-endquest a dozen times by now and would love if it were repeatable, but other totally hated the puzzle and jumping aspects of it. Many will argue that the IP reads StarWars not StarJumps or similar.
And as good as the bioncular and seed endquests are, they are "one shot" missions, you do them once and then they are over. Such content is faster consumed than produced and as such will create a lot of complaints rather than soothen some.

No...this game has horrible physics. HORRIBLE! Especially for platforming. As for puzzles, iirc, OPs were going to be much more puzzle fights etc...but it had to be cut down. As you can see from the EV Op.

Hero Engine capable of Swimming and underwater mechanics so they CAN add it.