Does anyone know a rough guide of how many seeds you need to complete all water critters. I know will depend how quickly you do stuff and how organised you are in advance. So just a rough guide 15 or 30-50 etc

There are so many Pond categories that it's very hard to put a figure on but you'll be looking at close to 100.

The underwater toadstool lasts a very short time...after fully dusted, it goes ready for harvest after a day and a half. And some of that time will be used up trying to get a mermaid attracted to plunge underwater. The rainbow of fish and bottom dwellers can use up quite a few underwater seeds to get the number of the required critters attracted to move along the challenge.
As you hunt for the required fish/bottom dwellers for the challenge requirements, you can pick up some seeds due to the spotting.
If one plants the mushroom needed for the watery seadragon on land and waits for it to go ready for harvest...that will save some underwater seeds....as well as when you have attracted the ten special rainbow of fish, you don't have to remain underwater to harvest the pondweed...the pondweed can be harvested on land. So that could save you some seeds.
Take a dip in the pond and attract a few fishy namesakes and see how that goes first to get a feel for things.

When I started Rainbow of Fish I think I only had 5 seeds. I ended up with 30 when I finished. You find find a lot as you play.