What template are you using for your marines?
Personally? 20w pure Marines + Support companies depending on situation. I'm sure there are better templates, though.
It's probably, strictly speaking, better to use 10w here. Much higher chance of reinforcing before your invasion is repulsed + can bring more support artillery to the party. Since line art takes a hefty penalty you probably want to skip the standard 7/2s, though if you've been making space marines anyway you can probably get away with it. 10w also decreases (but does not nullify) the need for signal companies, since each division has a base 7% (after radio) chance of reinforcing each hour, if you have 10 reserve units your chances of getting at least one in each hour are 1-(1-0.07)^10 ~ 51.6%. That said, if you include signal in the same position you get those odds up to 96%.
Marines are fairly decent units on their own (pure INF or special forces is one of the more underrated unit types in my opinion, especially with proper support companies, it won't beat a proper 7/2 1v1 but it's not made of paper either). If you're worried about casualties/manpower, a Field Hospital would not go amiss, since these divisions are going to be perfectly soft (and thus the AI's divisions are going to mulch you in a prolonged conflict - no one said Amphibious landings were easy).
If you're worried about being out of supply (like if you're going for TMIT's land next to port and attack into it strat) - you might consider maintenance to reduce attrition losses. Logistics is useless though, since it doesn't give you more out of supply grace.
Support Art + Support Rocket Art don't care about amphibious landings, so pick one or both for the soft attack boost.
I believe Engineers will completely remove the amphibious landings penalty, and Recon will help you get ashore.
Personally? 20w or 10w pure MAR + Support Art, ENG, REC, + Pick two of: Field Hospital/Signal/Maintenance/Support Rocket Art. I think Support Rocket Art and Signal are likely the best.
Like many things in HOI, it depends.