I would think you enter the water when it's slack/on the turn, and then get the advantage of the current as the tide goes out. That could knock 5-10 minutes per mile off your still water/pool time.
And apart from the elites and for most mere mortals, this is one to be enjoyed. For me, spending 2-3 hours face down in a river is pointless and not worth the £75 ticket price. Taking a bit of time to enjoy the swim, company and scrolling countryside? Yeah. That's worth the ticket.