"Ea" is a largely archaic word still used in some dialects to mean a river or watercourse. The Online Etymology Dictionary mentions "ealand" as a term formerly used to mean a watery place or meadow by a river. Are there any similar derived forms of "ea" still in use?
The OED has "aa" (stream, watercourse) as a cognate of "ea", but not as a derivative. "Aa" is a borrowing from Norse.
The "i" of the word "island" (originally spelt "iland" or "igland", with the g having a y-like quality) is also cognate with "ea", although not derived from it. (The "s" was later added through association with the word "isle".) The OED explains:
Old English ígland (íegland), íland, Anglian égland = Old Norse eyland, Old Frisian eiland (Middle Dutch, Middle Low German eilant, Dutch, East Frisian eiland), a compound of Old English íeg, íg, Old Norse ey (Norwegian öy), Old Frisian ey ‘isle’ + land n.1 The simple íeg = Old High German auwa, ouwa, Middle High German ouwe, German aue, au, corresponded to Gothic type *ahwiô, aujô, a substantivized feminine of an adjective derived < ahwa ‘water’ (Old Saxon and Old High German aha, Old Frisian and Old Norse á, Old English éa), with sense ‘of or pertaining to water’, ‘watery’, ‘watered’, and hence ‘watered place, meadow, island’. A cognate compound frequent in Old English was éaland, lit. ‘water-land’, ‘river-land’
Ultimately, though the relationship is more remote, "aqua-" is cognate as well.
As for "eagre", the OED doubts its relationship to "ea" ("The conjecture which connects it with the Old English éagor, égor, occurring in comb., apparently with sense ‘flood, ocean’, is untenable, because the Old English g in such a position would have become y in modern English").