I happened upon this tip on how to make floating candles from tealights, and its so easy. It’s important to seal off the bottom so the candle doesn’t sink.
I also suggest making sure your bowl doesn’t have a ‘lip’ where the heat of the candle can cause cracking. This happened to me once in a ritual, where we heard an audible ‘crack’ then the distinct sound of the bowl of water ‘peeing’ water everywhere. Talk about distracting!