I think Bulkhead could use a shift, in that its weapon should switch from Plasma Cannons to Gauss Cannons, for reasons I'll explain below.
Intended Usage Change
As it currently stands, as far as I've seen it used and used it myself, Bulkhead exists in a strange spot where it is useless against almost everything except choice units, and static defenses that do not outrange it or provide damage reduction. This, I find, makes it a relatively useless unit, despite the unit's capability to be terraformed for more range. As for why, this is because Buoy is already quite effective at hitting units Bulkhead is capable of hitting in the first place, while other units fulfill the Bulkhead's other opportunities effectively.
Bulkhead cannot qualify as mobile artillery, because as artillery goes, the Bulkhead has atrocious range despite being cheap, as well as suffering the drawbacks of being slow, forced to stop moving to fire, and having quite avoidable shots. Admittedly, Bulkhead is
not an artillery unit, but a fire support unit that rarely has the opportunity to support in meaningful ways.
The intention of switching Bulkhead's weapon to a Gauss Cannon is to ensure that it hits more consistently, allowing it to actually fight skirmishers with some degree of success, while giving it another place it can be useful, which is the water. By allowing Bulkhead to hit underwater, this would give it the unique position of being the only currently affordable mobile Gauss-using unit. Additionally, this would allow Bulkhead to support Amphbot vs Ships match-ups, as well as Amphbot sea mirrors, which I believe are either too one-sided or tend to stagnate into whoever stacks more Archers before switching factories.
This may also be a problem with sea itself being perhaps too balanced, or unbalanced, but that is irrelevant for the purpose of this suggestion.
Obviously, this would also allow Bulkhead to fire while underwater, which could threaten to massively upset unit balance in sea combat, but I think this would at least be worth considering, because as it stands Bulkhead has severely limited opportunities to be of use, let alone shine, on land or sea.
Stat Consideration
Bulkhead's current weapon's stats are 600 Range, 165 Damage, 1.90 Reload Time, 87 DPS, 320 Projectile Speed, and 20 Area-Of-Effect. Under the assumption it did change to a Gauss Cannon, I do not think that these would need to be drastically changed, other than Projectile Speed, considering the cost of a Bulkhead vs a Gauss Turret, and that static defenses are typically stronger for cost.
However, in exchange for it's newfound underwater capability, I believe the underwater HP Regen ought to be nerfed to a mere 10 Water Regen at the 30 elmo depth. This would also have the added affect of synchronizing Gauss closed HP Regen and Bulkhead 30E Depth Regen. As well, I think it would be optimal to make exchanges elsewhere, in regards to cost/speed, because whether or not it's viable on land or sea, a Bulkhead is simply a very slow option that many players won't leave open if they can help it. Another option could be HP, as Bulkhead is quite tanky, running 1540 HP, enough to survive exactly one Phantom shot.
In terms of weapon comparison to the current Gauss Turret, which has similar stats on a higher cost unit, other than health/damage reduction, I am of the opinion that assuming this change was enacted, Bulkhead should have very slightly decreased range, around 580, while GT has 560, and Placeholder has 575.
Conclusion
Bulkhead is a unit that currently, despite it's niche, is not useful due to other factors interfering in its moments of opportunity, and having lots of abusable issues, especially in sea. For these reasons, I think it could use a bit of a rework, including a shift in weaponry and stat allocation.
Please add more thoughts, and/or tell me I don't understand Bulkheads well enough :D