Can I legally taxi a seaplane in a canal or channel?
I would advise checking out the FAA's seaplane handbook for more information. In general local ordinances tend to be the issue here and not the FAA's regulations.

You can find the right of way rules in 14 CFR 91.115 


  § 91.115 Right-of-way rules: Water operations. 
  
  (a) General. Each
  person operating an aircraft on the water shall, insofar as possible,
  keep clear of all vessels and avoid impeding their navigation, and
  shall give way to any vessel or other aircraft that is given the
  right-of-way by any rule of this section.
  
  (b) Crossing. When aircraft, or an aircraft and a vessel, are on
  crossing courses, the aircraft or vessel to the other's right has the
  right-of-way.
  
  (c) Approaching head-on. When aircraft, or an aircraft and a vessel,
  are approaching head-on, or nearly so, each shall alter its course to
  the right to keep well clear.
  
  (d) Overtaking. Each aircraft or vessel that is being overtaken has
  the right-of-way, and the one overtaking shall alter course to keep
  well clear.
  
  (e) Special circumstances. When aircraft, or an aircraft and a vessel,
  approach so as to involve risk of collision, each aircraft or vessel
  shall proceed with careful regard to existing circumstances, including
  the limitations of the respective craft.


There is also a note from the hand book, 


  Pilots are encouraged to obtain the USCG Navigation Rules,
  International-Inland, M16672.2D, available from the U.S. Government
  Printing Office. These rules apply to all public or private vessels
  navigating upon the high seas and certain inland waters.


Although you should not expect your average boater to understand or even be aware of the FAA regulations on this. As such caution should always be used. Keep in mind that in most if not all states you don't even need a license to operate most personal sized boats (under 65 ft. generally). 

You will need to check with your local municipality (who ultimately has jurisdiction over the water way most likely). Unfortunately they may have local ordinances that are more intended for land based aircraft but may incidentally apply to you. Things like noise abatement may also stand in your way, keep in mind a plane is significantly louder than most boats.

This FAQ covers a lot of really good questions about where you can and can't land and other seaplane related topics.

Of interest to you may be Question 2 


  2. Where can I keep my seaplane?
  
  A seaplane can be kept at a certified seaplane base with permission, or at an  airport if it is equipped with
  amphibious floats. If you want to keep your seaplane "off airport",
  however, you will need to explore the issue with your state
  aeronautics office, local government and the owner of the waterway.
  Some specific problems include mooring restrictions imposed by the US
  Army Corps of Engineers, seaplane base licensing requirements imposed
  by state governments and zoning regulations imposed by local
  jurisdictions.


In this kind of situation I would personally seek out a local lawyer that specializes in aviation related matters (a call to your local GA field may get you a name). Unfortunately many local authorities don't always understand the regulations and many don't even know its with in their power to regulate such operations since the cases tend to be few and far between. In other cases older forgotten regulations may have relegated them selves to a dusty file cabinet that no one remembers. Other local sea plane owners may also be of help here. 

It is also worth considering if you have the width to actually pull this off. Even the biggest boats I have been on (in and around the 55ft. size) are generally no wider than around 15ft. at the widest point. A Cessna 172 has a 36ft. wingspan which is something worth thinking about.