As for locking the Y-valve vs. locking the seacock, I did some digging just last week and found this:
TITLE 33 - NAVIGATION
AND NAVIGABLE WATERS
CHAPTER I - COAST GUARD, DEPARTMENT OF HOMELAND SECURITY
SUBCHAPTER O - POLLUTION
PART 159 - MARINE SANITATION DEVICES
subpart a - GENERAL
159.7 - Requirements for vessel operators.
(a) No person may operate any vessel equipped with installed toilet facilities unless it is equipped with: (1) An operable Type II or III device that has a label on it under 159.16 or that is certified under 159.12 or 159.12a; or (2) An operable Type I device that has a label on it under 159.16 or that is certified under 159.12, if the vessel is 19.7 meters (65 feet) or less in length.
(b) When operating a vessel on a body of water where the discharge of treated or untreated sewage is prohibited by the Environmental Protection Agency under 40 CFR 140.3
or 140.4, the operator must secure each Type I or Type II device in a manner which prevents discharge of treated or untreated sewage. Acceptable methods of securing the device include (1) Closing the seacock and removing the handle; (2) Padlocking the seacock in the closed position; (3) Using a non-releasable wire-tie to hold the seacock in the closed position; or (4) Locking the door to the space enclosing the toilets with a padlock or door handle key lock.
(c) When operating a vessel on a body of water where the discharge of untreated sewage is prohibited by the Environmental Protection Agency under 40 CFR 140.3
, the operator must secure each Type III device in a manner which prevents discharge of sewage. Acceptable methods of securing the device include (1) Closing each valve leading to an overboard discharge and removing the handle; (2) Padlocking each valve leading to an overboard discharge in the closed position; or (3) Using a non-releasable wire-tie to hold each valve leading to an overboard discharge in the closed position.
And this from Florida:
FL Statutes - Title XXIV Vessels Section 327.01 Short title.
327.53 Marine sanitation.
(1) Every vessel 26 feet or more in length which has an enclosed cabin
with berthing facilities shall, while on the waters of the state, be equipped with a toilet. On a vessel other than a houseboat, the toilet may be portable or permanently installed. Every permanently installed toilet shall be properly attached to the appropriate United States Coast Guard certified or labeled marine sanitation device.
(2) Every houseboat shall be equipped with at least one permanently installed toilet which shall be properly connected to a United States Coast Guard certified or labeled Type III marine sanitation device. If the toilet is simultaneously connected to both a Type III marine sanitation device and to another approved marine sanitation device, the valve or other mechanism selecting between the two marine sanitation devices shall be set to direct all sewage to the Type III marine sanitation device and, while the vessel is on the waters of the state, shall be locked or otherwise secured by the boat operator, so as to prevent resetting.
(3) Every floating structure that has an enclosed living space with berthing facilities, or working space with public access, must be equipped with a permanently installed toilet properly connected to a Type III marine sanitation device or permanently attached via plumbing
to shoreside sewage disposal. No structure shall be plumbed so as to permit
the discharge of sewage into the waters of the state.
(4)(a) Raw sewage shall not be discharged from any vessel, including houseboats, or any floating structure in Florida
waters. The operator of any vessel which is plumbed so that a toilet may be flushed directly into the water or so that a holding tank may be emptied into the water shall, while the vessel is on the waters of the state, set the valve or other mechanism directing the sewage so as to prevent direct discharge and lock or otherwise secure the valve so as to prevent resetting.
(b) All waste from Type III marine sanitation devices shall be disposed in an approved sewage pumpout facility.
(c) All waste from portable toilets shall be disposed in an approved waste reception
(5) Every vessel owner, operator, and occupant shall comply with United States Coast Guard regulations pertaining to marine sanitation devices and with United States Environmental Protection Agency regulations pertaining to areas in which the discharge of sewage, treated or untreated, is prohibited.
Notice that for type 1 & type 2 there is a need to lock the SEACOCK vs the requirement with type 3 to lock the VALVE.
I wish they had used the term Y-Valve to be clearer. But it is pretty obvious they mean the Y-valve vs the seacock when talking about the type 3 system.
As I understand this, a type 1 or 2 is a Lectrasan type system and a type 3 is a holding tank system.
So Lectrasan systems need to be locked at the seacock but holding tank systems need only be locked at the Y-valve.