Thanks to Meme.
Ramses Sukot Migdol Pi-Hachirot Sea Eitam Shur-Desert Mara Eilim Sin-Desert
This is not a map, just a schematic presentation.
1. ויסעו מסוכות ויחנו באיתם בקצה המדבר
Going from Sukot to Eitam one has to cross the sea
2. וישובו ויחנו לפני פי-החירות בין מידגול ובין הים
They returned and parked ahead of Pi-Hachirot between Migdol and the sea. Going back from Eitam to Pi-Hachirot they had to cross the sea in the opposite direction than before – they returned.
3. ויסע משה את ישראל מים סוף ויצאו אל מדבר שור
To get to the Shur-Desert they had to cross the sea again.
4. ויסעו מרתה, ויבואו אלימה, ויסעו מאלים […] אל מדבר סין.
We see here that BNE YISRAEL crossed the sea three time, one of the crossings was by going back toward Egypt.
Now, let us look at the description in SEFER BAMIDBAR.
1. ויסעו […] מרמסס ויחנו בסוכות. ויסעו מסוכות ויחנו באיתם בקצה המדבר
Going from Sukot to Eitam they had to cross the sea.
2. ויסעו מאיתם וישב על פי החירות […] ויחנו לפני מיגדול
As we saw in SHMOT, Moshe took the folk back from Eitam to Pi-Hachirot. As there, also here they had to cross the sea in the opposite direction.
3. ויסעו מפי החירות ויעברו בתוך הים המדברה […] במדבר איתם ויחנו במרה. ויסעו ממרה ויבואו אלימה
Now they have to cross the sea, but this time it says that they crossed it by going into the sea, and they eventually reach Eilim. Here they did not go as far as the Sin-Dessert as mentioned in Shmot, but instead:
4. ויסעו מאלים ויחנו על ים סוף. ויסעו מים סוף ויחנו במדבר סין
Here we have another march back toward Egypt. It is not mentioned in Shmot. Did they cross the sea here or just stopped at the sea-shore? All the same, after looking at the sea for the last time they leave Yam-Suf and reach the Sin-Desert, which they did in Shmot going directly to it without returning to the sea.
What we see is that the journeys of Israel, after leaving Egypt, was forward and backward. They cross the sea and reach the desert. Paroh says: the desert closed on them (not the sea). When Paroh dicides to go after Bne Yisrael, Moshe takes them back to the sea. What happened there at KRIAT YAM SUF we all know.
What is surprising is that in Shmot we have three crossings of the sea, one of them in the opposite direction. In Bamidbar we have also the three crossings, but before getting to the Sin-Deseret they go back to the sea once more.
At KRIAT YAM SUF we know how Moshe and Israel crossed the sea. I wonder how were the other sea crossings done?