Hey, found a solution for this, which was driving me crazy as well…
Search for this node in all BP - “Add Spline Mesh Component” (for me, it was used in a function within my river spline BP Construction script).
Click this node and adjust collision settings there in the Details panel.
This post tipped me off.
Using these settings, my character could pass thru shallow water and get a hit result from visibility for the surface material.
Seems the collision for a spline mesh can’t be adjusted any other way.