[Legbreaker]

I'm working on the assumption the bulk of the marine force would land in the vicinity of Stegna, with primary objectives being the village & bridges at Rybina (where the HQ would set themselves up), Vistula river crossing at Dworek, Nowy Dwor (both town & bridges), and the bridge at Jazowa.
Secondary objectives would be the bridge at western end of the island of Sobieszewo, the lock at the south east corner of Sobieszewo island, and the ferry across the mouth of the main channel at Mikoszewo.
These objectives give the 8th ID and any follow on units two prepared routes with crossing points to choose between.
Once secured, moves could be made on Elblag, probably with assistance from the newly arrived (hopefully) 8th ID.

Although the east and west flanks of the marines would be protected by rivers, there's still a LOT of open flood plains on their south flank through which the enemy could attack. Much of the 4,000 strong force would need to be allocated to defending the three river crossings (Dworek, Nowy Dwor and Jazowa) and patrolling between those locations and further south. As these three crossings are the most likely axis of advance for follow on units, they'd absolutely HAVE to be held.

With approximately 1,300 troops assigned to supporting roles (HQ, logistics, transport, artillery, engineering, etc), and one regiment (say 600 men) defending the secondary objectives around Sobieszewo Island, that leaves only about 2,100 including the recon and armoured elements to defend a front with few helpful terrain features of about 22 kilometres. Also, units would need to be positioned/patrol along the flanks to ensure the rivers weren't crossed by enemy forces and the combat units attacked from the rear (or the supporting units attacked at all).

All up, the Marines would have a front to defend of approximately 40 kilometres which blows out to nearly 60 kilometres if Elblag is taken (no natural barriers to the east, south or north, although there does appear to be some old ancient earthworks to the north).

With Pact Cavalry units in the area, and the HQ of the Baltic Front known to be at Malbork (Intel Estimate 142, June 2000), just 15 kilometres or so away from Nowy Dwor, the Marines can't afford to overstretch themselves.