Bite or Water gun on blastoise

I would say Ice Beam gives enough coverage for Giratina, and personally I prefer Water Gun over Bite because of the charging speed. I haven’t fought against Cresselia in Ultra League yet, as no one in my area is willing to power it up to get the most out of it.

Water gun is pretty good on Blastoise, you do need the CD Hydro Cannon though plus Ice Beam to get the most out of him. Hydro Pump just takes too long to charge & Flash Cannon isn't terrible, but again the charge time is an issue plus it doesn't hit Blastoise counters for SE damage like Ice Beam does.

I'd say it depends on the rest of your team's composition and whatever the PvP scene looks like in your area since both have their uses. Water Gun builds energy faster while Bite can shred through things weak to it.