If everyone wore life jackets, only 21-7=<<21-7=14>>14 people could fit on the raft.
For every person less on the raft, 14/7=<<14/7=2>>2 life jackets could fit.
For 8 life jackets, 8/2=<<8/2=4>>4 less people could fit.
21-4=<<21-4=17>>17 people could fit on the raft.
#### 17