Is it legal to use a handheld marine VHF transceiver from a small private plane to communicate with a boat? I can't find any material that directly addresses this.

The Coast Guard regularly uses marine VHF from rescue aircraft/helicopters, but from a private plane...not sure.

FCC Parts

Check out Part 80. Happy reading; it's a LONG part.

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73 de K7KBN

Yes, but it's use is basically limited to distress and safety. 5 watts max in the air (with some exceptions) and commercial fisherman can use 157.425 except near certain borders (Canada, etc.). Here's the whole applicable paragraph from part 80:

Sec. 80.379 Maritime frequencies assignable to aircraft stations.

This section describes the maritime frequencies assignable to aircraft
stations for simplex operations:

(a) Available frequencies:
Carrier frequency Conditions of use
2738 kHz (1)
2830 kHz (1)
3023 kHz (2)
4125 kHz (3)
5680 kHz (2)
121.500 MHz (4)
123.100 MHz (4)
156.300 MHz (5)
156.375 MHz (5)
156.400 MHz (5)
156.425 MHz (5)
156.450 MHz (5)
156.625 MHz (5)
156.800 MHz (5)
156.900 MHz (5)
157.100 MHz (6)
157.425 MHz (5)(7)

(b) The conditions of use of the carrier frequencies in paragraph (a) of
this section, are:

(1) For permissible geographic areas of operation see Sec. 80.373(b)(1). For
other limitations see Sec. 80.373(b)(7);

(2) Aircraft and ship stations may use 3023.0 kHz and 5680.0 kHz for search
and rescue scene-of-action coordination including communications between
these stations and participating land stations. Stations using these
frequencies must use J3E emission;

(3) Assignable for distress and safety communications between aircraft and
maritime mobile stations;

(4) Assignable for search and rescue between ships and aircraft. Stations
using these frequencies must use A3E emission;

(5) These frequencies may be used by aircraft stations when:

(i) The altitude of aircraft stations does not exceed 300 meters (1,000
feet), except for reconnaissance aircraft participating in icebreaking
operations where an altitude of 450 meters (1,500 feet) is allowed;

(ii) The mean power of aircraft stations must not exceed five watts;

(iii) Communications are limited to operations in which the maritime mobile
stations are primarily involved and where direct communications between the
aircraft and the ship or coast station is required;

(iv) Stations may use 156.300 MHz for safety purposes only;

(v) Stations may use 156.800 MHz for distress, safety and calling only; and

(vi) Use of 156.375 MHz by aircraft is not permitted in the New Orleans VTS
area specified in Sec. 80.383.

(6) The use of 157.100 MHz is limited to communications with stations of the
Department of Interior at Lake Mead, Nevada; and

(7) Commercial fishing vessels and associated aircraft may use 157.425 MHz
while engaged in commercial fishing activities except within 120 km (75
miles) of the United States/Canada border and Puget Sound and the Strait of
Juan de Fuca and its approaches, the Great Lakes, and the St. Lawrence
Seaway.

[ 51 FR 31213 , Sept. 2, 1986, as amended at 58 FR 44953 , Aug. 25, 1993]