400 Amp service to 225 amp and 150 amp panels
I'm concerned you're overthinking this.   You're not required to section out your 400A allotment to X for one building, Y for another etc. that totals up to 400.  You are allowed to &quot;oversubscribe&quot; the buildings, so long as they're behind breakers.
Also keep in mind, 400A is a lot of power. All-electric houses have long gotten by on 100A.
Build your system to have the versatility for any site to use full power
For simplicity and economy, I'd simply do the bog-standard &quot;two 200A panels&quot; that everybody does with 400A service.   House panel A and B.
However, I'd make house panel B &quot;special&quot;.  I'd use a panel with thru lugs. The thru-lugs are after the panel's main breaker, but otherwise just hotshot straight off the bus.
Off the thru-lugs, I would continue 200A wires the 20 feet to the bunkhouse. Hey, it's only 20 feet, the cost of 4 wires isn't going to beat you up too bad.   There, I'd fit a 225A (for the safety margin) main breaker panel.  The breaker's value doesn't matter; it's only there to be a disconnect, which is required for an outbuilding.
Is it an outbuilding? The minimum standard for &quot;not an outbuilding&quot; is &quot;connected by a breezeway&quot;, so you can skip the main breaker and go main-lug if there's a breezeway or better.
What this means, is house panel B and the bunkhouse share a 200A breaker.
From this panel in the bunkhouse, go ahead and feed the other buildings.  You can feed them with a breaker from bunkhouse's panel... Or if you want to carry full 200A onward to the next building, go ahead and do that - you can just &quot;tee&quot; off the feeder wires coming into the top of the bunkhouse breaker.  (that is to say, the 200A line to the garage does not need to be protected by the bunkhouse breaker, since it's already protected by the panel B breaker.)
Personally if I were me, I'd feed the dockside with a &lt;=50A breaker, and the reason is so I can make it GFCI.  I feel strongly that all wiring anywhere near water should be under GFCI protection, and that GFCI protection should be placed far from dockside, back at the supply point.  That way if something happens inside the dockside subpanel, or to the feeder wires, it too is protected. Electrical drownings are serious, and dockside is where that happens the most.
You won't be maxing out house, bunkhouse, garage and boathouse at once
This is the part where we get to &quot;oversubscribe&quot; a bit.  You're vanishingly unlikely to be welding in the garage at the same time you're running the A/C and cooking on a boat at the same time you're using hot water and cooking in a bunkhouse at the same time you're cooking and doing laundry in the main house.  There are only so many of you, after all.
So that means it's perfectly OK to potentially have 50A of boathouse load with 100A of garage load (Tesla fast charger) with 100A of all-electric house in the bunkhouse with 120A of stuff in the main house.  You simply are not capable of maxing them all at once.
However, back at the house, I would make careful choices about which loads I put in the exclusive panel A vs which in the shared panel B.  Have the shared panel contain loads like bedrooms, basement, living rooms, maybe garbage disposal and microwave - loads you are unlikely to &quot;pull hard&quot; while being active in the garage or dockside.  Loads that you realistically do pull quite hard, like A/C, heat pump emergency heat, water heater, range etc. go in the exclusive Panel A.