We get plenty of tidal movement in the river at Ballina where I foil. I seem to go better with the tide than against it. I realise the tidal movement is increasing my airspeed on the sail but I thought that sailing against the tide would increase the water speed over the foil, creating more lift. This doesn't seem to be the case.

Any ideas, theories etc?

Its fun surfing the small waves caused by the outgoing tide anyway & they often help me get up onto the foil.

Short answer, no. You are immersed in the medium so, unless you are anchored/tethered, etc so you aren't, you are essentially oblivious to what's going on outside of the water that surrounds your board.

Another way to think of it is an airplane. Airplanes take off into the wind to shorten their take off roll, right? As long as they are attached to the ground, the effect you describe occurs: the wind is already flowing over the wings and a lower speed is required. However, as soon as the plane leaves the ground, it doesn't matter. The airplane is immersed in the air around it and a head wind doesn't make it go faster or provide greater lift.

A low speed airplane in a high headwind can actually fly "backwards" relative to the ground. It's still going 45-50 kts but the entire air mass is moving backwards.