A Ring of Water Magic increases the effect of all Water spells by 50%, rounded down. So if a spell normally does 8 points of Water damage, it will do 12 points of Water damage if the caster is wearing a Ring of Water Magic. Wearing 2 Rings of Water Magic has no additional effect.
The artifact amulet Morgan increases the effect of all Elementla Magic spells (including Water) by 50%, and this stacks with the effect of a Ring of Water Magic, so that a water spell cast by a wearer of both Morgan and a Ring of Water magic has its effect increased by 125%.
Therefore, a Lloyds Beacon spell cast by a master of Water Magic with 12 points of skill in Water Magic will last for 84 days (skill x weeks). If he's wearing either Morgan or a Ring of Water magic, the spell will last 126 days (skill x weeks x 1.5). However, if he's wearing both Morgan and a Ring Of Water magic, the spell will last for 189 days (skill x weeks x 1.5 x 1.5).
The only other way of getting the spell to last longer is to increase the caster's skill in Water magic by putting more skill points into it.
Hope this helps!