Curly arrow mechanism for reaction between sodium borohydride and water?
Their mechanisms are easily searchable online.
This is from 2009 from art-xy.com:

Notice that the reaction would be different in the presence of metal catalysts:
$$\ce{NaBH4 + 2H2O -&gt;[metal catalyst] NaBO2 + 4H2}$$
This is from 2007 from chembook.co.uk:


Which reaction is more violent is already explained by Jan:

In one case [$\ce{NaH}$], you have free hydrides, which go ‘Oh my god, I see a proton, I’m going to jump at it!’ and in the other case [$\ce{NaBH4}$] you have hydrides thinking the same thing, but being pulled back by a boron saying ‘No, stay here, I need your electrons! I’m poor!’

Therefore the former is more violent.