I don't think a second battery would hurt. Redundancy is good when you can't walk home.

If your plan is to only run the radio while the engine is on, you can get by with 1 battery, since the alternator will always keep the battery topped off.

If you think you might want to run it while anchored and swimming or something, definitely get a second battery, and an isolator switch like this.

https://www.amazon.com/dp/B00144B6AE/ref=cm_sw_r_cp_awdb_imm_2NRQVS7VDB7Z62Z5PS28