Yes, multiple "outgoing heal effectiveness" effects will stack additively. If you combine Aquatic Benevolence with Rune of the Monk, outgoing healing will be increased by 35%.
I did some tests on this in Heart of the Mists by equipping a Rune of the Grove and traiting 0/0/0/6/0 for 1397 Healing Power, then observing the effects of my Soothing Mist on myself and others.
The base effectiveness of Soothing Mist was 150 HP/second on both myself and others. When I swapped the Rune of the Grove for a Rune of the Monk, the effect on others increased to 165 HP/sec. Adding the Aquatic Benevolence trait increased it to 203 HP/sec.
These numbers work out increases of 10% and 35% over the baseline of 150.