1)
What is the meaning of "v'Samti va'Yam Yado"?
1.
Radak: He will have power and authority over those who live on the sea shore and in cities on rivers. We find that Chiram Melech Tzor sent a gift to David via the sea - cedar wood, wall builders and carpenters - "Ki Hevi'u ha'Tzidonim veha'Tzorim Atzei Arazim la'Rov l'David" (Divrei ha'Yamim I, 22:4).
2)
What do we learn from "uva'Neharos Yemino"?
1.
Radak: The kings of Aram, who lived on the other side of the [Yarden] river, used to send a gift to David on the river - Kelim of silver, gold and copper. Also, David took from all the cities in Ever ha'Nahar when he fought the kings of Aram.