Morning Dawney,
Grrrrr...I answered your questions last night...it took time as I hunted out some links for you too. Anyhoo...when I hit the 'submit' button.....something went stoooopid and the whole post was lost.
So here I am giving it another go as I am pleased my thread is generating some interest.
OK.....
There are far more undersea volcanoes than there are on the surface of the planet...obvious really given the 70% thing.

However, there are anywhere between 25-35 volcanoes erupting at any given time. The issue is not more volcanoes, but the volume of their ejecta. The volume of ash/chemicals that the volcanoes are spewing does
appear to be increasing. Here is a link to a website which will give you a constantly updated list of what's rumbling, emitting or in full blown eruption...
www.volcanodiscovery.com/erupting_volcanoes.html
Now, this leads to your next question about their significance. Besides the obvious danger that volcanoes present, the increase in their ejecta can affect our climate. As an ongoing event with multiple volcanoes erupting we hardly notice these changes. So to give you an idea of how volcanoes affect our climate, lets consider an extreme event.
The year before Krakatoa erupted (I am making the assumption that your good self and others know of the significance of that particular eruption) A supervolcano called Mount Tambora erupted. This particular volcano makes Yellowstone look small by comparison! When this volcano erupted in 1815, a VEI 7, the volume of emissions (ash/tephra etc) gave us what has been called "The Year Without Summer", basically almost a 'nuclear winter'. It even affected wine production as far away as France...basically the wine was shit!
en.wikipedia.org/wiki/Mount_Tambora
Now, as I said...the ammount of volcanoes in full blown eruption mode isn't necessarily increasing, but the volume of crap
(good crap btw in the long term) is increasing....so as the geological clock ticks at a very, very slow pace compared to our human clock, the accumulative effect does change things, imperceptably some would say if you just look at the 'maths' in our own lifetime. But if we get one of the Big Boys blowing...then we are in serious trouble.
Also, when you get big eruptions happening in populated areas, we always tend to forget that, besides the danger this poses to the local population, it also affects the viability of the surrounding agricultural land and infrastructure...sometimes for genarations.
Hope this helps a bit, and I will do my very best to answer any other questions you might have

Jane