Between these two, the 550D (Rebel T2i) is the better body to use. It’s broadly similar to the 450D/XSi, but gives you higher resolution and video capability, which can matter for surfing and general flexibility. Neither body is dramatically ahead in handling; they’re from the same entry-level Canon line.

The good news is lenses are not locked to one body: both cameras use the same Canon EF/EF-S lens mount, so any EF or EF-S lens you buy will work on either camera and can remain useful if you stay with Canon APS-C DSLRs.

For bird photography, a telephoto zoom is the sensible place to start rather than jumping straight to expensive super-telephotos. A lens in the 55-250mm class is a practical budget-friendly starting point.

For underwater surf photography, the body matters less than finding a suitable housing made for that exact camera model. Since housings are model-specific and can be expensive, that may influence your choice as much as the camera specs do.

So: keep the 550D if possible, invest first in lenses you can keep using, and only buy a housing if you can find one that fits your chosen body well.