As I have never played that game, how will the Astartes fleet work?
They have Battle Barges as battleships, will they be tanks, absorbing damage while closing in to board the enemy ship?
Astartes fleets are divided into rapid strike vessels (escorts), Strike Cruisers (cruisers), Battle Barges (battleships), and possibly a Fortress Monastery (picture a space station, only twice the size and several times the firepower). Some chapters, particularly First Founding chapters or ones with large holdings maintain ships outside these classifications, but not many.
All Space Marine ships have improved boarding actions; first, Space Marines, second, even the ratings on Space Marine ships are considerably more armed, better motivated, and better trained than Navy ratings. It's to the point where it was actually considered impossible to board a Battle Barge or Fortress Monastery, as no opposing ship had nearly enough crew to fight several companies worth of Astartes. (Hit and run, which is what this game means by boarding, is actually okay.)
Striker Cruisers and Battle Barges have IN prow armor, except it's on every facing.
Lances are in short supply. They are replaced by short-range Bombardment Cannon; fixed-forward for Strike Cruisers, turreted left-front-right for Battle Barges. Bombardment cannon hit like lances, but automatically inflict a critical hit. (No idea how that'll translate; automatic "fire" maybe?)
Space Marine ships have boarding torpedoes and regular torpedoes. They can choose which to fire, like Orks can in the current game.
Thunderhawks can act as assault boats or fighters (in the tabletop, on the same sortie, but I doubt that here). They are also naturally more durable, and have a chance to avoid being removed when they are hit or engage the enemy. This will probably translate as being hard to stop with fighters or turrets.
Space Marines are expensive comparatively, obviously.