From Rule 35 of the USCG Navigation Center for Inland vessels:
Rule 35
(a) When power-driven vessels are in sight of one another and meeting or crossing at a distance within half a mile of each other, each vessel underway, when maneuvering as authorized or required by these Rules:

(i) shall indicate that maneuver by the following signals on her whistle:

one short blast to mean “I intend to leave you on my port side”;
two short blasts to mean “I intend to leave you on my starboard side”;
three short blasts to mean “I am operating astern propulsion”.
(ii) upon hearing the one or two blast signal of the other shall, if in agreement, sound the same whistle signal and take the steps necessary to effect a safe passing. If, however, from any cause, the vessel doubts the safety of the proposed maneuver, she shall sound the danger signal specified in paragraph (d) of this Rule and each vessel shall take appropriate precautionary action until a safe passing agreement is made.
(d) When vessels in sight of one another are approaching each other and from any cause either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle. [Such | This] signal may be supplemented by at least five short and rapid flashes.
Remember that “Port” and “Starboard” always refer to your vessel.
Our thanks to the AGLCA and the Burkes for this good advice and reminder to review the Rules of the Road!

We live on the waterway just south of Mile Marker “0” and in the last week there have been two occasions of “five” whistles by passing tugs pushing barges. The Southbound tugs sounded signals at approaching Northbound cruisers. Each event was started by the tugs sounding appropriate passing whistle signals . . . the cruisers ignored the proposal, and the five whistle signals followed. Vessels passed very close, fortunately there were no mishaps. Norfolk is a very busy place and as the summer season is soon to begin, and, with the looper meeting taking place here in Norfolk, please know that we urge every boater to monitor channel 13 in the Harbor and to talk early to approaching large ships and tugs. Call them on 13 as soon as you see them. They will work with you and like very much to hear you on the radio, channel 13. They then know you see them, have a better feeling regarding your competence, all resulting in understanding and a reduction in everyone’s level of concern. Be sure you have a clear correct understanding of the Rules concerning One Whistle and Two Whistle passing. Be safe out there.
Charlie & Bonnie Burke
M/V SONATA (GB42 Ocean Marine Portsmouth)

IMHO it is not wise to use actual horn signals. Commercial vessels rely on radio communication and they are unlikely to be able to hear your puny horn’“use VHF channel 13! Other pleasure boaters may be able to hear your horn, but 90+% won’t know what it means and it will just cause anger/confusion. Instead, first try contacting the other boat on the VHF’“usually channel 16 works for pleasure boats. Failing that, assume the other boat may do something unexpected and don’t try to cut it close in a narrow channel until you know it is safe to pass. If you do get a horn blast from another boat it often means `pull over to your starboard, I’m coming through’ irregardless of what the actual horn signal means. So if you hear one blast from astern and you see a fast boat coming up don’t immediately swing to port! The chances are very good they don’t understand what one horn blast means.
John Kettlewell

I have noticed that from Louisiana to Florida’s west coast, most recreational boaters, and all commercial skippers use the terms `one or two whistles’ according to the rules. On the east coast from Florida to NC, most recreational boaters say I am passing you on the port or starboard side, which can be very unclear in an overtaking situation. Is this a regional thing? Also, I have handled court cases involving boating accidents, and disregarding the proper sound signals will certainly work against you in that venue.
Berwick Duval