Hey I just played through the current build of the game, and my biggest takeaway is the lack of telegraphing on boss attacks. The only way I knew that the boss was going to attack was because I died a couple times to it before and learned how close I had to be to bait an attack. It would be fantastic if there was some sort of animation or sound that alerted the player that an attack was coming -- even if only a couple frames before. Also maybe a different way to let the player know that the tentacle is going to spin around, because I was not able to figure out which attack would trigger on my playthrough.
Another thing is that while I understand that you want to show the player that the boss fight is still going, and that it is just underwater, often the bosses shadow will cover up falling rocks, leading to the player being hit when they can't see where the rocks will land.
My final critique is about the dodge. In its current state, it does not feel usable. On the fights where I actually beat the boss, the only time I used the dodge was to move the last couple of inches out of the phase3 charge attack. Avoiding the tentacles with the dodge is a death sentence and I hope that this is remedied somehow -- maybe lowering the time it takes to get up?
All in all I had a great time with it, and I'm excited to see what bosses are coming next. Finally a question: When the game does fully release, will the Early Access owners receive the full copy, or will we have to buy it again?
edit: after reading some other replies I would like to add that I used mouse and keyboard, did not use targeting, and feel that aside from the lack of telegraphing boss attacks, the whole fight felt very fair.