Really comes down to the application.
If you're talking about water heaters used as household ones are....heating separate systems, and used occasionally for brief periods, the 125 amp panel should be fine.
If these are used together, as in supplying hot water to a production process, and use continuously, than you have to treat the loads as continuous, and have that 25% "buffer' added in to both your panel and the wire supplying it.
(Use the KW ratings of the heaters for your calcs).
