Sea guards are a mix of spearman and archers, the way i see it in a unit of 15 seaguards the only time you should use them over spearmen is when you have a 1.5k+ game. since the table should be big, there is a higher chance of being able to shoot more oftern. although the SG they are good for weakening the opponant whilst you advance towards them (the problem is the +1 to hit)
and wht do you mean spearmen too weak? 4+ Armour save, attack in 3 ranks, always strieks first. i would be lost without them!