Is connecting a 16A plug to 10A sockets for an electric water heater a problem?
It depends on the power rating of the water heater.  From an earlier question, I see that you live in China, so your electrical supply is 220&nbsp;VAC/50&nbsp;Hz.  That means that the maximum power draw on a 10&nbsp;A circuit is 2.2&nbsp;kW (220&nbsp;V * 10&nbsp;A).  In general, you should draw less to allow a safety margin, if the water heater uses more than about 2&nbsp;kW you should get it rewired because you'll be coming close to the maximum rating of that circuit.

Note: I'm not even considering the presence of other appliances on that bathroom circuit (lights, electric razors, electric toothbrush chargers, etc.).

Based on the additional info in the question, if you only use the 1000&nbsp;W and 1500&nbsp;W modes on the heater, the existing wiring should be sufficient.  If there's a way to prevent people from using the highest setting (like mounting the whole heater in a box where it can't be changed inadvertently), you should be OK.