I'd be inclined to agree. The beaches around there all seem to be stone as well as packed. And frankly, while not a fan of bringing a powered boat into a beach with people in the fist instance, Brighton Beach strikes me as possibly the worst beach in Britain to even try that with. It's full of stones, stoners and the professionally offended operating on overtime to seek out and assault lesser human beings such as the despised 'normals'.
I'd probably prefer to find an estuary to launch from instead, such as Shoreham.
Having said that, I would personally bin that bit of coast of departing from Sluff and instead head to somewhere a little further West such as Chichester Harbour where there is real sand, some non tidal slips, mellow people and a reasonable speed limit?