High voltage amplification of a waveform generator output
Ultrasonics are driven by peculiar means. If you also need to receive (I assume you don't) you also need to worry about the T/R switches.
There are two kind of transducer: resonant and wide band. A resonant transducer is made for working at exactly one frequency, like sonar sensor or atomizers. These are usually driven either in self oscillation mode (they work like crystals or, more exactly, ceramic filters) or with some pseudo class C amplifier: you load an inductor and then make it discharge thru the transducer in a transient, it will then oscillate at his own frequency (given the right LC network around it, of course).
These are also called 'pulsers' since you give a pulse and wait for it to do it job (maybe it get received by something else or do some mechanical work).
25MHz seems more a frequency used by imagers (that's why I asked if you needed the receiver too), and you drive them mostly like big speakers (which they are, in fact).
The old school called for a matching transformer: you made your signal at, like 12V and a substantial current, then put it to a step up transformer (often specified or even supplied by the transducer manufacturer) that outputted the 100-800V required (yes, there are piezo stack working at more than 800V).
These day is often 'simpler' (quotes are a must here) to have a +50V -50V (or whatever) split power rail and then make a suitable class B or AB amplifier. 25MHz is usually tough as RF so you need to shop for an RF transistor (luckily there is the 27MHz ISM band near); power isn't really a big issue since they make these BJT (maybe some MOSFET too) in all the sizes.
Depending on your particular application there are also ICs dedicated for the application. For example Supertex (now Microchip) has a wide range of pulsers and T-R switches, look in their portfolio to see if there's something you can use (for example the MD2131 is a somewhat high end driver, but there are simpler parts too).