So maybe I'm not exactly the best at interpreting poems, but I'll give it my best shot...
What I think this means, is that sometimes something will be remembered for a short period of time, before being forgotten. Sometimes you may be famous, but then as the years pass you are forgotten. Think of all those One Hit Wonders. Spenser is writing a women's name he most likely loves in the sand, but it is washed away by the tide. The tide symbolizes how the women feels, when Spenser says "and eek my name be wiped out likewise" means they'll probably both forget about each other. But Spenser wants something immortal that won't be erased. When he says "but you shall live by fame" and "in the heavens write your glorious name" he is saying that she won't be forgotten to him even though she'll probably move on.