Welcome to the forum.
Here is the Owner's Manual for your 2018 270SDX.
According to page 21 the seacock is within the ski locker hatch floor. It is possible / likely for the seacock to be fully covered by a floor panel that must first be fully removed, after the locker is emptied.
WRT to regulations, the discharge must be inhibited. On my 2000 380DA I close the seacock and remove the handle. Others use nylon wire ties or padlocks.
The regulations say this: While operating a vessel in an EPA designated no discharge zone, flow-through devices are only permitted if adequately secured to prevent discharges of all treated and untreated sewage. For example, closing the seacock and padlocking, using a non-releasable wire tie, or removing the seacock handle are considered to be sufficient in most cases. For short voyages, locking the door to the head with a padlock or a door handle key lock is another acceptable method. For vessels that routinely operate in no discharge zones a Type III MSD is recommended. For more information see 33 CFR 159.7 and 40 CFR Part 140.
