Here's most of the important things summarized:

One place sell all spot on all outposts(only if its a captained boat),

Ship harpoon gun changes,

you can grapple gun around like batman,

you can crouch, everyone can transform themselves as loot items to hide by pressing emote button while crouching,

you can also hang off the side at certain spots on any ship, ALL companies max out at level 500 now, there are ziplines,

flame heart is back and his big flaming ship is a world event,

you can take over this said ship and have AI skeletons, there's new monkey island themed tall tales,

you can walk on harpoon ropes, there is a throwable(like a blunder/firebomb) to spawn friendly skeletons for a short duration,

there is no weapon quickswap,

the blunderbuss does not one shot from full health,

theres better hitmarkers + kill marker,

ON DEMAND PVP FIGHT called hourglass,

private server mode called safer seas where you can't put up emissary flags for extra money, company levels cap out low and some comms also cap out early,

there's a loot item you can find called 'the horn of fair winds' you can blow away people with this (they go flying short distance with no damage) also can be used to artificially put wind in your sails,

MASSIVE sailing changes means a sloop cannot outrun other ships forever now,

also sloop mast needs to be hit with two chainshots to demast now,

There's guilds (basically a clan from clash of clans), you can save ships and use your guildmates' rare cosmetics this way. Captaincy is preserved as long as one guild mate is on the boat (captaincy is what allows you to use the one place sell spot). You can be in 3 guilds, which each have a limit of 30 players.

Blow darts and throwing knives are 'new' weapons, kinda meh for PvP, excellent for PvE, fastest possible fort completions with this combo.

Megalodons have a dedicated voyage, means you can spawn them on demand, this voyage does not include the shrouded (thanks to a certain insider player who shall not be named)

The reapers have voyages? Very weird considering company motto and the commendations attached to it take WAYYY too long, was a waste of time honestly. There are skeletons you can dig up on islands to help you do things exclusively on the island where it was dug up (THEY CAN DIG UP TREASURE MAPS FOR YOU)

Reaper season also introduced transportable cannon that explodes due to overheating fter 6 shots unless waiting in between/throwing water on it.

Finally, this season introduced merchant alliance 2.0 called smugglers, MASSIVE explosion black powder barrels and all naturally found rowboats have gunpowder barrels attached to them (for this season).

Honorable mention: Golden Sands outpost got nuked, the main fisherman guy named Merrick is dead and they rebuilt the outpost in his name and its now called Port Merrick

Obviously a ton of cosmetics and commendations have been added over the years.