Scope here for an e-boat, the sort that turns the engine off at traffic lights er, locks, then restarts as soon as you waggle your 'elum, if you'll pardon the expression.
I'd say that that sort of use, while waiting/in locks etc must come under the category of being under way. However, while you're under way, you are heating your water and charging your domestic batteries, but most of the fuel is going into turning the prop, so maybe a 60/40 split there is quite a generous concession by Excise Men.
I don't keep complete records but do pencil in on the receipt what the split is and put it in the log too (now that's a self-serving statement if ever I saw one) and generally guess as to what use this particular fill will be put to. As we spend a good few weekends on or near (within 400 yards) the marina but run the engine for a couple of hours daily for water/lecky, I give that 10/90. If I know that, say, half of the fill of say 100 litres will involve cruising, then 35/65 is the way to go. I reckon I can justify that, especially as I record the engine hours daily - all in pencil, of course!