But how many of these are viable options for a 1st or 2nd lv party? They will likely be forced to enter combat, or run for their lives and wait out its duration. It seems like against them, the summoned monster will be the equivalent of a proper monster. And if the triton opts to summon something else...
The PCs aren't meant to encounter 1 solitary triton at level 2--they're meant to find a company (2-5), squad (6-11), or band (20-80), and 90% of them are meant to be mounted. So a single CR 2 triton as an ECL 2 encounter isn't what the rulebook is counseling, here; they're supposed to be encountered
en masse and with animals to ride upon as parter of a larger, higher ECL group.
Let's see...
A company of 4 tritons each riding porpoises makes for a total ECL of 7. If they act as the Monster Manual says they should and summon their CR 3 water elementals, then run, the party is left with 4 CR 3 elementals, for an ECL of 7--and the elementals have the weaknesses described previously, so it's actually on the easy side of ECL 7.
I'm actually a bit surprised at how well that math works out!
Those weaknesses I described earlier apply to anything they summon, not just elementals, so even if they all summon a CR 4 Giant Crocodile, that's still just an ECL 8 encounter--less, really, because of the
dispelling and 7-round duration and whatnot. I expect a party of four 7th-level adventurers will have the wherewithal to deal with whatever the tritons summon quite nicely.