Cthulhu is extremely important for Water teams. The AoE -DEF is indispensable on trash, the Charm lands at a decent rate (30-50%) and greatly aids in delaying overdrives (one of Water's key strengths), she has a free -ATK buff, and the overdrive reduction is useful though unreliable. However, for teams with Ryu-Oh and Vine, she's somewhat dispensable. Keep in mind that Sol only reduces damage by about 15%. That matters on bosses, but for trash Cthulhu's AoE -DEF provides a damage boost that far exceeds the loss.

Having 3 -ATK doesn't really matter. With Cthulhu, you'd kill the first trash mob in Rank 4 in 2-3T anyway. Triton's main utility in a group with Cthulhu is her Combo attack down. Her self-heal also makes her pretty durable as a frontliner.

I use Cthulhu and Triton in my Rank 4 team, though I suppose if I didn't have Gaia in reserve I'd be more afraid of the final boss's Raging Overdrive. Instead, I'm mostly worried about triple attacks taking out someone important early on. If I were to spend my next Miracle ticket on my Water team, I'd replace Sol with Nike though. Snow Raphael is a lower priority for me since I want Sniper Shot to deal with the trash. Bringing in SSR Nike would allow me to sub out Sol, Gabriel, and Triton and bring in Shiva and Poseidon instead. The trash can't combo attack me to death if they're dead, and Blind from Apocalypse should be sufficient to deal with the other phases.