Okay, I played quite a bit this weekend. I'm on my 3rd ship - no special ships, just the normal upgrade path.
Ship combat does not have the depth and complexity to it that Black Flag has.
What it does have, however, is the essence - aiming, firing, sailing, very very close to the Black Flag experience.
I think some of the unlocks like fire barrels and so forth are behind drops and/or quests that I haven't seen yet.
The initial batch of things, there aren't things like heavy shot, ramming armor, fire barrels, swivel guns, and mortars. There is a front mount cannon and there are side mount cannons. I know for a fact that there is a legendary ship that can drop fire barrels - he was dropping them as I killed him. I don't have it yet.
It's a little thing, but I like that the crew starts cheering in Black Flag when you return to the helm.
The sea shanties are recorded songs, not chants by sailors.
There is what I consider a very large feature missing currently. There isn't a controls option to always steer the ship based on the current heading, rather than follow the camera. As you circle the ships you are attacking, depending on your camera, it can reach a point where your port direction becomes starboard. It's an immediate change, so it's jarring. Imagine you're looking at the side view of a ship. If you are 1 degree "left" (so your camera view moves to look slightly rearward on the ship) pushing left on the stick does not move your ship to the port side. It moves to the starboard side. If you go just another degree in the other direction, the direction the ship moves flips the other way.
They also drastically need to add outfit options to your characters. I find myself leaving the complete dive suit on 24/7, unless I know I'm going into a boss fight. Speaking of boss fights, there are a few bugs with a large one in the expansion - I won't spoiler it, but it cost me 300k having to replace my lost large ship. Ah well. My advice - don't travel to the center part of the lower portion of the map unless you are prepared for a boss fight. It will be triggered on one of the missions from the pirate island that you have to take over - after you complete that and you're "following the sound of the roar" - avoid that area unless you are prepared for it. Approach it from the north - don't island hop and explore like I did and get caught off guard. Also, when you see the pop up for the boss fight - any exploration you do will get wiped out when you "leave the area" and you will be given the option to retry or abort. This typically isn't an issue with most boss fights, as they are contained to a reasonable sized room. This boss fight covers like 10 square nautical miles.
Overall - really, really enjoying it. I'm spent a lot of time making money through businesses. I didn't have too much from my pre-expansion save, but I'm somewhere in the realm of 12M on hand right now, and I spent 1.2M on a medium ship, 6M on a large ship, and the costs to outfit them. I also wasted some special items needed to purchase a special rear deck, only to realize I need the LARGE one, and I bought the medium. I guess I could outfit a medium ship, but I'm past those now, with no need to ever return. I'll have to change back over to doing normal missions to level up some of my guys and collect resources other than money. I now have enough funds to also start doing the investment missions, which need millions to do. I plan on shutting down the shops, which will cost about 20k per day in leases, but I think I have enough to cover that.
If you're just starting out, collect as much wood via Greymane missions as possible. You will need a lot. Also chop trees, you want perfect timber - you need a ton of it. Like when you see a grove of large trees together, take a few minutes and collect. You can get a bonus to get extra wood logging axe from the Berghren(sp) timber camp when you can lease that one. You want the extra wood, even if your first lease is also a timber camp. Finally, to raise the prestige (not the right word but you will figure it out) of the business, you can place the small graymane flags really, really close together. Just lump them in the corner to get the business up to max prestige, which will let you raise prices and have the customers be cool with it.
