"Dragon purgatory! I thought I had killed you!"
"You did... But I've returned now, more powerful than ever! Muwahahaha!"
That said, dragon turtles are significantly better than their non-cheloniiac brethren, if only because they have a balanced HD-threat ratio.
One bite attack and two claw attacks allow for a decent attack routine with a Mouthpick weapon, and full BAB, 27 strength and Huge size don't hurt there either. Breath Weapon is pretty good at wiping minions, and all metabreath feats apply to it. At worst, it's a 1/encounter 12d6 damage boost with Quicken Breath.
Capsize is a bit of an oddball ability, and its usefulness highly depends on how frequently ships will be faced. It's unlikely to affect LA either way though. Dragon Turtles also have immunity to Fire (neat), Sleep (meh) and Paralysis (semi-nice).
In the end, I'd say +1 LA fits. The subpar dragons may have made it hard to see, but dragon HD are very good when actually gotten with a decent melee creature. Dragon Turtles are very strong melee combatants, and a small LA shouldn't hurt too much.