Supposing I'm a regional carrier and my routes never pass over water bodies (sea or lakes which are over a certain size). Am I obliged to carry life jackets on my plane?
Assuming you are asking about civilian flights operated under FAA rules, part 91 rules (14 CFR §91.205(b)(12)) advise that approved and serviceable flotation devices are required to be carried for each occupant during flight where such flight takes place over water outside power-off gliding distance to shore. "Shore" has a specific definition.
If the aircraft is operated [for VFR flight during the day] for hire over water and beyond power-off gliding distance from shore, [it must carry] approved [and serviceable] flotation gear readily available to each occupant and, unless the aircraft is operating under part 121 of this subchapter, at least one pyrotechnic signaling device. As used in this section, “shore” means that area of the land adjacent to the water which is above the high water mark and excludes land areas which are intermittently under water.
These provisions are extended to VFR flight at night and IFR flight in 14 CFR §91.205(c) et seq.
Part 91 subpart F stipulates the requirements for large/turbine-powered multiengine operations (where such rules exclude flights for the purposes of common carriage). In summary, §91.509 requires approved flotation devices when operated more than 50 miles from the nearest shore, and liferafts, survival kits, etc for flights operated more than 30 minutes flying time or more than 100 nautical miles from the nearest shore.
Most "commercial" flights in large aircraft are further regulated in terms of equipment by sections under part 121, part 125, part 135, etc. with similar equipment requirements to those noted above (links take you directly to relevant sections of the code for extended overwater operations under each part).
If operating a commercial air tour compliant with part 136, and such tour is to take place over water beyond minimum gliding distance in a non-compliant aircraft except for the purposes of takeoff and landing, then the code in 14 CFR §136.9 is more strict: occupants must wear life jackets until at least the period when the flight ceases to be over water.
(a) Except as provided in paragraphs (b) or (c) of this section, the operator and pilot in command of commercial air tours over water beyond the shoreline must ensure that each occupant is wearing a life preserver from before takeoff until flight is no longer over water.
(b) The operator and pilot in command of a commercial air tour over water beyond the shoreline must ensure that a life preserver is readily available for its intended use and easily accessible to each occupant if:
(1) The aircraft is equipped with floats; or
(2) The airplane is within power-off gliding distance to the shoreline for the duration of the time that the flight is over water.
(3) The aircraft is a multi engine that can be operated with the critical engine inoperative at a weight that will allow it to climb, at least 50 feet a minute, at an altitude of 1,000 feet above the surface, as provided in the Airplane Flight Manual or the Rotorcraft Flight Manual, as appropriate.
(c) No life preserver is required if the overwater operation is necessary only for takeoff or landing.
In summary, for FAA aircraft operating solely over land, the regulations noted above and by Ralph J's comment proscribe that the required equipment to be carried is minimal.
Of course, what the letter of the law says, and what's a good idea to carry, may in some cases be different things -- I'm thinking here about good practice to carry additional life preserving equipment when operating a GA aircraft for an extended time over water.