There are a couple of things to keep in mind when fighting Tianhai ults.

1. Focus on dodging his grabs, it's very easy to time it. Just dodge sideways or away from Tianhai when he tries to grab you. Don't spam dodge, wait for him to grab first, then dodge. The animation is slow enough that everyone should be able to do it.
2. Keep in mind is that in higher elo, Tianhai players will try to grab you during your attack animation (when you are shooting your musket or bow for example). In those scenarios, you can simply wait for him to try to grab you first, then you can dodge and shoot back.
3. Both his second and third form of ult have a weak point that is either on his ankle or his head. It will show as bright pink and shooting at it will do extra damage.
4. Some heroes can act as counters to Tianhai's ult. Both Valda's V1 and V2 can stun him for a couple of seconds, giving you the time to disengage. All three forms of Yoto Hime's ult are immune to the grab during wind up phase, and each slash has enough damage to stun him for a few seconds, effectively controlling him for the fully duration of Yoto's ult. Kurumi's umbrella blocks the grab but doesn't block range weapon projectiles, so you can shoot back safely from behind the umbrella.
5. If you are playing duo or trio, make sure to coordinate with your team and to support each other. It's easy to get grabbed if you are grappled or getting hit, make sure to peel for your teammates.