Hello,
Where do I find pondweed to plant?
I need to start with all that underwater but can’t seem to find what I need.
Many thanks

It's under Pond Mythological. 
Check the special behaviour - e.g. for Watery Seadragon:
Special Behaviour
Every 5th Watery Seadragon spotted in your garden will plant some Red Pondweed (but only if there are some "Red Rocks" available in your Garden to plant it in!)
The red rocks are available in the "pond" section of your pot shoppe.

First thing you need to do is grow each mushroom to be RTH so it will visit, before you go underwater - good luck 😊

You have to put out the correct colour of rocks for the pondweed you want, then set up for the underwater myth which corresponds to that colour (you can see that information in the Wildlife Guide).  The underwater myths need RTH mushrooms so you'd need to get your mushroom RTH before going underwater.

So if you are starting with the red pondweed, you get the Fairyland Agaric mushroom RTH, buy the red rocks from the pot shop, and go underwater to attract the Watery Seadragon.  The next multiple of five you attract plants the pondweed in the rocks.

Too slow ;)

Thank you both!!