Not sure about exactly what you want (under the FAA system): a 'multi-engine sea' (MES) certificate at one of PPL, CPL or ATP levels, or
A 'multi engine land' certificate + either/both 'single engine sea' (SES) and 'multi-engine sea' certificate(s) - also at whatever level you want.
A Multi-engine land certificate can be done just about anywhere. Pick an airstrip with a school & chances are they'll have a twin in which to do the training & test.
SES & MES schools is rather more limited. There's a a couple of float schools near Orlando, FL. Jack Brown's Seaplanes can definitely do the SES and, I *think*, have a multi on floats to do the MES.
There's also two float schools based at Flagler County airport (X47) a little north of Ormond Beach. One only does SES - using a C180 amphib - and the other does MES only using the amphibian Twinbee.
The advantage using the amphibs is that you can get the land certificate at the same time as the floaty one.