The N210 is daughterboard based. That is, the N210 itself is a fancy network connected digitizer (ADC/DAC + FPGA + ethernet interface) and it requries a front-end card for analog interfacing. In most cases, this is some type of direct-conversion radio receiver, but there are also simple frontends which just provide a direct interface to the ADC and/or DAC.
In this case, it sounds like the N210 in your system is using the LFTX and LFRX daughterboards, which as you have correctly guessed stand for Low Frequency Transmit and Low Frequency Recieve.
These particular frontends are basically just opamps + filters to give the USRP 30 MHz of TX/RX bandwidth.
I don't understand where the conversion to baseband for message parsing would be coming from.
Obviously, we don't know your particular system, but if you have only 30 MHz or so of analog bandwidth (even that seems high for an underwater communications system), the baseband conversion is probably happening either in the FPGA or purely in software. Have a look at digital downconversion to get an idea of a typical architecture.
That is assuming that you are modulating a carrier in the underwater communications system. You may just be using the baseband directly, in which case you can process the real signal directly or do something like use a Hilbert transform to convert it to a complex baseband signal.