I am always surprised by the amount of myth there is around diopters. Just to recap your 60mm lens in water becomes 75mm. However because of water magnification your reproduction ratio is not affected. Now if you have a minimum focus distance of 25cm and you out a diopter with length of 165mm you will have a working distance between 145 and 125 (from the back of the lens) say you have 165 for simplicity divide 250/165 and you have 1.53 so a +6 diopter gives you 2.53x or 1.53:1 if your lens already was 1:1. A +10 diopter would gives you 3.5x or 2.5:1. There will be small variations and the working distance will usually be less from the front of the close up lens (as the lens height is included in it). Depending on the amount of magnification you get depth of field is reduced proportionally. So less depth of field with a stronger diopter. Ultimately you need to work out what lens you need and how small are your subjects. With your lens and a *6 your frame height will be 16mm which is enough for a Pygmy seahorse. With a 10 you are looking at less than 1cm frame Height so you can now fill the frame with your Pygmy on a portrait
