I come from the benign neglect school of parenting, but this bothers me somewhat. Yes, 10 years old is old enough to be expected to be sensible and stay safe, and to take responsibility for a younger sibling for a while. But I think 3 hours is too long a time to expect the 10 year old to be in charge when out by a lake (had the 10 year old been left in charge at home for that length of time, I'd have been less worried).
Also, 10 is still young - young enough to get carried away having fun, or to fall out with the younger sibling (and the younger sibling is equally or even more likely to get carried away or have an argument), in which case things can go from fine to disaster very quickly.
All it would need would be for a game of tag to go wrong and for one of them to fall in the lake and get into difficulties. 10 years old is too young to be expected to be a lifeguard, let alone 7. Or they have a row and the little one runs off and the older one can't find him/her - cue huge panic and upset on both parts.
For their father to say that he had to go and train for 3 hours sounds supremely selfish unless he's a professional athlete - imo he should have either found someone to have the children for an hour or two so he could train, or should have postponed his training session.
I have activities that I am committed to - like my art group, where I have my closest friends up here and where I get a lot of pleasure, but I wouldn't dream of putting my need to do my painting ahead of the boys' needs.