What is the closest I can realistically have my spring to the ocean?
When, as a kid, I went to the sea during summer, it was common knowledge that, in some places along the coast, there were springs pouring fresh water directly into the sea.
One could tell it by both the sudden drop in temperature, especially with calm waters preventing mixing (the surface was chilly, while the bottom was warmer) and by the different appearance of the waters due to the different density. While I could see for quite a distance with my bare eyes open underwater, when I went into the spring zone the water looked kind of foggy and seeing through it was much harder.
In the generation before mine such places, in particular where the water was shallow, where used by shepherd to supply their herd with drinking water, which was otherwise impossible to find in the surrounding Mediterranean shrub.
The geology of the place where I grew up is made mostly by limestone surface layers, with clay like layers underneath. The interface between limestone and clay is where water accumulates and flows. There was no visible spring, rather a diffuse percolation through the rocky coast.