@ ChasB46, your explanation is faulty

The names you have are correct, WNA and so on

Saltwater and/or temperatures have absolute nothing to do with draftmarks.
The draft marks are set for the regional weather areas,therefore winter north atlantic or summer and tropical etcetc. It is because the righting levers become higher with more freeboard and therefore WNA needs less draft because one can safely assume that in WNA a lot seawater would come in bad weather on deck and reduces the stability of the vessel, the lower draft compensates for that. In the tropics where bad weather is not so much the factor the draft can be bigger, hence tropical mark
When you load in Canada and sail to will say Manaus, Brasil (Fresh water) then you are not reaching the draft mark assigned for that area. However if you load the vessel in Manaus on permitted draft then you will be over mark arrival Canada.
Worldwide seewater has 1.025 , This changes only with areas in so far as for example in coastal waters rivers flow in and dilutes the saltwater. Baltic has a very low salt content for example,,also med-sea has lower

Deadweight = the total displacement of the ship @ loaded summer draft minus the ships weight !!!!

brgds
capt ted

The deadweight of a vessel is the total weight of cargo, bunkers, dunnage, provisions, fresh water, ballast (if any), stores and spare parts, expressed in tonnes of 1,000 kilogrammes, which a vessel can lift when loaded in salt water to her maximum draught. In winter salt water is at its most dense and as a convenient time the safe loading mark is measured in relation to winter. In summer water is warmer thus less dense so can support less tonnage. If not adjusted between winter and summer the ship would sit lower in water - beyond maximum safe draught.
Also note marks are set at Tropical Fresh Water, Tropical, Fresh Water, Summer, Winter and Winter North Atlantic. For example if you load a ship in Canada lakes in winter to its maximum Winter North Atlantic draft and sail to tropical area of world then it would sit lower in water or sink! Blame Samuel Plimsoll.

Could someone explain Summer DWT? I've read various articles (Wikipedia, Googled it) but it's very confusing. Can it be simplified? Why does it have to be a Summer measurement? Is anything measured for Winter?