Most nonbasic lands that produce blue mana arenâ€™t Islands. For example, Glacial Fortress isnâ€™t an Island. Some nonbasic lands (such as Irrigated Farmland from the Amonkhet set) do have basic land types printed on the type line and may be Islands.
Kumenaâ€™s Speaker gets only +1/+1 from its ability, no matter how many Merfolk and Islands you control.
Because damage remains marked on a creature until itâ€™s removed as the turn ends, the damage Kumenaâ€™s Speaker takes during combat may become lethal if you no longer control a Merfolk or Island later in the turn.