Hi - are there any places on Murano where I can just wander up and grab a water taxi without having to book it by phone in advance?
Good question that I don't know the answer to. But you do know that they are ferociously expensive, right?
There might be some waiting near the main docks where the vaporetti (water buses) also stop. However why bother? The vaporetti can take you back to the historical center of Venice with a fraction of the cost (€7 or even less if you wisely buy the daily pass for 24 or 48 or 72 hours etc). If you take the water taxi expect to pay about €40.