see images here and here.

In To His Love, the river Severn is the idyllic river where he used to be "under the blue" with his love, and is no longer. 

This beautiful river is the longest in of all Britain and has the most voluminous water flow in Wales (Wikipedia). This beautiful river also has an association with young death, as in To His Love. a masque called Comus by Milton, there is a nymph named Sabrina who drowned in the river (Wikipedia). Also, River Severn is an Estuary, where fresh and seawater mix. (Severnkeeper).

Sources:

https://en.wikipedia.org/wiki/River_Severn

https://www.britannica.com/place/River-Severn

http://severnriverkeeper.org/about-the-severn-river