Any of the channels on the Modem-M64 will work at close range, when getting closer to the range limit (200m) the choice of channel matters.
Channel 1 has a bit lower signal strength which gives shorter range
Channel 2-3 has the strongest signal (and is attenuated least in salt water) which is why they are recommended.
Channel 4 is slightly non-linear, it should work but probably has shorter range and might be more unpredictable.
Channel 6-7 are good but will be attenuated more in salt-water than channels 2-3 which means shorter range.
I’m guessing that any of the channels will work fine at 60-80m, so try to find the channel that gives you the least interference.