Here is a quick and simple guide in choosing optics for surveying whales and dolphins

I'm going to start with the simple advice that if you are serious about whale watching you should consider spending as much as you can afford.  Yes, the price of Swarovski is sooo much more (and probably three times the price of other good brands - that are not 3 times poorer) but more expensive brands usually come with high quality glass that lets in more light, superior lens coating, weather proofing and doesn't distort the image especially around the edges.

From the shore

If you are intending a watch from the shore one tip is to choose a place that has some height. For example if you are standing on the shore at sea level and your eyes are around 170cm from the ground the distance to the horizon is around 4.7 Kilometres. But if you are standing on ground that is 5 metres above the shoreline the horizon is now 9.2 Kilometres enabling you to survey much more ocean.

Magnification in Binoculars is usually expressed as Magnification X Diameter of the lens (in millimetres) So 10X50 means 10 times magnification with a 50mm Objective Lens.
Magnification from 8 to 12 times would be good. Bigger is not always better as 12 times means you would be surveying a much narrower area then the 8. However, the image will be bigger.

You should also consider the relative Lens measurement. A small lens like 8x20 would have poor light. As a rough rule of thumb if you take the lens size and divide it by the magnification the result should be more than 4.

E.G. 8x28 would be 28/8 = 3.5 (poor light)
Whereas 8x42 would be 42/8 = 5.25 or 12x50 50/12 = 4.2 both much better light.

Scopes

It must be said that from land a scope on a tripod will allow you to see much more detail. Again, this will be a much smaller slice of the sea so the scope to zoom in on something you spotted with the binoculars is a great pairing. I would suggest the formula described above should exceed 2.5

So a scope that is 30x85 would be 85/30 = 2.8

My preffered kit

Swarovski BTX 115mm lens scopeAfter much testing over the years I find the Swarovski EL 12X50 binoculars paired with the Swarovski BTX 115mm lens absolutely outstanding. The scope is only 35 times but has a binocular eyepiece with makes long viewing sessions much easier than squinting through one eye and 115/35 = a whopping 3.29 (so great in low light)

From a boat

Needless to say there is no point in bringing a scope on a moving boat! Regarding binoculars I would not use anything more than 8X magnification as the movement of the boat would make any stronger magnification impossible to keep steady.

Personally on boats I use the Swarovski EL 8.5X42 binoculars