Space Marine Fleets
Reflecting its role as a mobile strikeforce, Space Marine Chapters will have several vessels under its command at any time. These ships are all designed with one purpose in mind: getting its compliment of Astartes to where they were needed, as soon as possible. Once there, the vessels will support to the Space Marines on the ground (usually via Orbital Bombardment), and keep on standby to retrieve the ground forces once its mission was completed.
Compared to its counterparts in the Imperial Navy, Space Marine warships are fast, heavily armored and shielded. Armed with short-ranged weaponry, Astartes ships are designed to close in to enemy warships, hit them with Bombardment Cannon broadsides, and then unload its deadly cargo of Astartes, either via Boarding Torpedoes, or Teleport assaults.
For a typical Chapter, its fleet of warships is considered a vital part of its forces, but for a "Crusading" or fleet-bound Chapter, their fleet will literally be their home. For the latter, the loss of each ship will be keenly felt, as a part of the Chapter's history will be lost with it.
A "typical" Chapter fleet will be composed of two or three Battlebarges, enough Strike Cruisers to support ten Companies of Space Marines, and any number of Escort ships. There are exceptions of course, with fleet numbers of First Founding Chapters varying wildly.
For the most part, ships of a Chapter's fleet will be crewed by Chapter serfs, who are far better trained and motivated compared to a Navy vessel's crew.
Space Marine Ship Types
The largest warship that a Chapter is capable of fielding, a Battle Barge is typically any large warship capable of supporting and deploying three Battle Companies of Space Marines, as well as vehicles and other equipment. Reflecting the need for self-sufficiency, many will feature their own Apothecarion, Reliquaries, Librariums, and even areas to train new recruits to the Chapter.
A standard Battle Barge will be armed with forward-facing Torpedo launchers (for standard or Boarding torpedos), mid-ranged Macro Weapons Battery broadsides, the powerful Bombardment Cannons, along with Launch Bays to deploy Thunderhawk Gunships and (lately) Stormtalons.
Making up the bulk of Astartes fleets, the Strike Cruiser is essentially a Battlebarge in miniature, capable of supporting a single Company of Space Marines and its equipment. Despite being faster and more agile, it is also armed similarly, with Torpedo Launchers, short-ranged Weapons Batteries, Bombardment Cannon, and Launch Bays.
Unless a Chapter mobilizes in force, the Strike Cruiser will be the most commonly-seen Space Marine warship seen in any given warzone.
An Astartes fleet will be supported by any number of Escort ships, which would help screen the larger ships from Torpedo attacks, bombers, or attacks from enemy Escorts.
There are generally two types of Escorts: the bigger and better-armed and armored Frigate, and the smaller but more agile Destroyer. While relatively fragile, they are much more faster compared to the bigger Space Marine ships to compensate.
The Ordinance-boat of the Frigates, the Gladius is usually the most numerous ship in a Chapter's fleet. It also transports small strike forces of Space Marines for missions that don't justify the need for a Strike Cruiser.
A Gladius is normally armed with Macro Weapons Batteries.
A dedicated Lance Frigate, the Nova-class comes armed with short-ranged Macro Weapons Batteries and a Lance as standard.
It's important to note that, in the original boardgame, the Nova was the only Space Marine ship armed with Lance weapons. Except the first version of the Seditio Opprimere, but no one talks about the Seditio Opprimere.
The equivalent to the Cobra-class of the Imperial Navy, the Hunter Destroyer is armed with a pair of Torpedo Launchers and a Macro turret.
In Battlefleet Gothic
Space Marine ships have a much narrower focus compared to Imperial Navy units, in that almost all weaponry available are short-ranged. This forces a Space Marine player to get their ships up close and personal to do damage. Because of their good armor, they can certainly survive these short-ranged firefights.
Due to the points costs their ships, a pure Space Marine list will be outnumbered most of the time. Choose your fights wisely.
Space Marine vessels have high Leadership scores by default. They also have better rolls for Boarding Actions and Hit and Run; likewise anyone trying to do Boarding Actions against Space Marine ships will be penalized instead.
In Battlefleet Gothic: Armada 1 & 2
Space Marine ships are the heaviest-armored units in the game, bar none, with armor on all sides equivalent to the Armored Prows of Imperial Navy vessels. They also have better Leadership, and are more effective in Boarding Actions.
On the flipside, they're also much more expensive points-wise compared to their Navy counterparts, and have a far more limited ship list.
To give one an idea, the Strike Cruiser, which was just a Light Cruiser in the tabletop game, had to be given three variants. These "variants" just divvy-up the default armaments of an in-universe Strike Cruiser to create Carrier, a Torpedo boat, and a Lance-boat, respectively. All this, just to fill out the "regular" Cruiser size category.
Much like the specialist game, due to the short weapons ranges of their ships, Space Marine vessels are brawlers by necessity. However because of their excellent armor values, they can easily survive these firefights, which is always a good thing.