Can I fill the upper void in an I-beam laid on its side as a seawall with concrete?
With a concrete thickness of only about 100mm, it is not easy to get it right — that is pretty close to the limit of what is reasonably possible. Some points to consider:


The concrete recipe is critical, and you should hire professionals for this by buying the fresh concrete ready to pour directly into the form. Assuming that's sea water (containing lots of chlorides), the concrete will extremely exposed to freeze/thaw issues and the mix needs to be exactly right. This aspect is not diy-territory in my opinion.
Drain holes won't do anything until after the concrete is heavily cracked. Don't bother drilling those.
As for coating, you probably won't have to do anything. The optimal steel surface for cast-in steel items is usually no coating at all, so you shouldn't add anything. Whether the existing paint job is suitable for being in contact with fresh concrete is hard to tell, but that is unlikely to be a problem.
You will need to reinforce the concrete to limit the cracking. Cracks will allow water and chlorides to penetrate into the concrete, which is what will eventually cause the damage. You can't avoid cracks entirely, but with appropriate reinforcement, you can ensure that the cracks will be small enough to not a problem. The appropriate amount of reinforcement will depend on the exact concrete recipe, so hire an engineer to do the calculation. As an approximate guide, I suggest 12mm bars per 200mm in both directions (must be welded fabric) with a minimum cover of 40mm. The requirements to the concrete cover on the rebars is what makes this thickness problematic. And a minimum cover of 60mm would be a better choice. The cracks will primarily come from shrinkage and thermal movements in the concrete during the hardening phase.
You should give the upper face a small slope to the side, so water can run off. 1:20 will be excellent but even 1:50 will increase the lifetime dramatically.
It might be better to extend the concrete off the sides of the beam, so the steel and concrete joint doesn't meet on the upper face, as that will expose the joint to water. If you don't do that, I suggest placing 20mm by 20mm spacers at the top of the concrete/steel interface, and later fill that joint with a bitumen based joint sealant. This is in order to prevent water from seeping down along the interface between the two materials.
Ideally, you would ensure that the concrete isn't able to slide along the very smooth steel surface (from shrinkage and thermal movements). The standard method would be welding headed shear studs onto the steel beam, but with a thickness of only 100mm, that isn't really possible. Any small steel items welded onto the beam will help, though, for example small 20mm by 20mm by 20mm steel cubes spaced regularly. You should expect to have to remove the coating on the steel locally before welding, as otherwise you may get various toxic fumes.