IN GMS there are tutorials which you can download from their official website. from there you can get a complete data set for simulation of saltwater intrusion.

Thanks Subhajit Dey.

While I do not know elevation of the screened interval, can I create wells other than what GMS tutorials have created for the aquifer?

yes I simulated seawater intrusion for a mountain to coast area located in north part of iran. it was really time consuming as data is not available and process of simulation is really laborious. SEAWAT was used for this simulation.

Search for Henry Problem. It is a good start up example of saltwater intrusion modeling. You can find the problem details in the SEAWAT document.

Thanks Wissam Al-Taliby.