Procrastination's a pain in the ass. Anyway, my take on a possible overhaul on the Echoise line. Any small numbers refer to footnotes at the end of the post, to explain stuff better.

Current Dex Entry

First of all, the base stats. They're pretty good, but the terrible Attack sticks out like a sore thumb. Sure, it's Special-based, but you almost never see 'Mons with five very good stats and one horrible one that isn't Speed, with the exception of the more gimmicky ones. The starters especially aren't like this. An attempt to balance the line a little:

Echoise
HP: 53
Atk: 40
Def: 50
SpA: 57
SpD: 54
Spe: 60
BST: 314(1)

Mermeidon
HP: 68
Atk: 51
Def: 65
SpA: 76
SpD: 70
Spe: 73
BST: 405

Dolphure
HP: 89
Atk: 65
Def: 83
SpA: 103
SpD: 93
Spe: 97
BST: 530(1)

There. The line still has the same feel, but now it isn't as minmaxed. And as an added bonus, Tackle won't be borderline useless when a low level Echoise is up against something that resists Water.

On to levelup movesets. Dolphure's stats make it both a great Special sweeper as well as a good tank, but it's only decent as a wall, as its defenses are only above average. We need to get rid of the redundant moves, reduce the amount of support moves (since walling isn't Dolphure's main thing, it should be more of a secondary concern), and spread the different move types around more (too many attacking moves at low levels, too many status & support moves at mid-high levels).

("--" means it starts with it, and can be re-taught by the Move Relearner. "xx" means it can't be learned by this form.)

Moves: (Echoise/Mermeidon/Dolphure)
Tackle --/--/--
Tail Whip --/--/--
Water Gun 7/--/--
Supersonic 11/--/--
Water Sport 14/14/14
Water Pulse 18/18/18
Soothe Song 21/22/22
Cascade 24/xx/xx
Light Wave xx/26/26 (2)
Windwhistle 28/32/32 (3)
Take Down 32/35/xx
Signal Beam xx/xx/36 (4)
Serenade 36/40/42
Purify 39/43/46 (5)
Hydro Pump 43/48/52
Water Wall 48/53/xx
Wrath xx/xx/58 (6)
Tail Glow xx/xx/65

Actually, that turned out better than I was expecting. I even snuck in most of the more defensive moves as lower form exclusives. Originally it was to give them something in place of Light Wave, Signal Beam, etc., but then I ran with it when I realized that it would help people that wanted to breed their original, and make the kid more wall-based. A wall build wouldn't be as good during the game as a sweeper with better type coverage, and keeping your starter in its first form would be counterintuitive so early on. But someone that already beat the main game, or at least is really far, and wanted a Wallphure wouldn't have to worry about keeping it in a weaker form for so long.

I feel like Tail Glow really helps Dolphure out. The move actually makes a lot of sense on the dolphin, and Dolphure can really be a threat against opposing teams when it can effectively boost its base SpA to 206 in one turn.

You may or may not have noticed that Dive isn't in there any more. Yes, it did fit, but there was a problem with it: it's an HM move in Topaz. Gamefreak has always avoided having HM moves learned naturally in the games the moves are actually HMs. The exceptions are Waterfall, which gets the grandfather clause as it's a Goldeen line exclusive and has been since 1st Gen, when it wasn't an HM, and Whirlpool, which wasn't an HM in D/P/Pl when the 4th Gen movesets were created.

So...opinions? Possible edits?

1. I looked at the canon starters' stats on Smogon, and I noticed that in relation to the others, the Grass starters had slightly higher BSTs in their first forms and slightly lower ones in their final forms, the Fire starters were the opposite, and the Water starters were in between them in both forms. Interestingly, every one of them had a BST of 405 in their second forms. Echoise and Dolphure had the stats of a Grass starter, so I edited them a little to be more in line with the Water ones.
2. Light Wave's power is middle of the line, and I felt like Level 32 was a little late to learn it. By moving it down to 26, Mermeidon gets more type coverage at an earlier time, and gives players a taste of what's coming up for it.
3. It seems like all of the starters, at some point, have a move that is learned right away at one evolution. With Light Wave moved down, I needed something there. A move that is basically an upgraded Supersonic seemed to work, even though the line can't really do anything with that lowered Defense. Then I decided to make it available to the lower forms as well.
4. Works with Dolphure, as it can already channel light through its tail; Signal Beam isn't much of a stretch. It helps its type coverage, gives Ludicolo (a possible Dolphure counter) something to worry about, and, more importantly, gives Dolphure a way of supereffectively hitting Dionare, since as of right now, Grass resists Light, making Dionare neutral to it.
5. Moved it up because of how powerful it is. I mean, it's basically a party-wide Esuna.
6. To be honest, I'm unsure about this one. I mean, it helps Dolphure a lot, but it might be too much. If we take it out, move Tail Glow down to 58.