Sounds pretty good to me, I think you sing better than me and I sing live for parts of our band's set.
I think it would be easyer to give you critisisms if it was just you singing and nothing else (no bass or backing song). I hate singing while not playing myself so I'd find it hard. You do seem to stay in tune well though.
I think the key is just practice, I've improved a fair amount lately just from singing along with songs.
I'd suggest giving a few more a go, ones I find fun to sing and play which aren't too hard and of a similar style are:
NOFX - Stickin in my eye, Bob and mabye Seperation of church and skate.
Screeching weasel - Hey suburbia
Bouncing souls - Manthem
Jawbreaker - Boxcar
If your interested in singing and playing ska stuff I'd start with the verses of Leftover crack - Gang control or Big D and the kids table - Wailing paddle as they're not too tricky.