Hola D K,

For this part: "...the worst part was the wind, since he was afraid of capsizing." we've got two separate actions; the first one using the preterite "was / fue" is indicating that the fact that the wind was awful is seen as something complete, it happened then and we see that "windy time" as something that started and ended at some point, as opposed to "...era el viento", which would be more descriptive, i.e you can imagine in your head the scene of a windy day in action (instead of something that had an end). Then, with "...tenía miedo." they are describing how they were feeling during that period - if they'd said "...tuve miedo" we would see that action as something that did happened but with a definite end.  

You may find this lesson useful:

Imperfect vs Preterite

Un saludo cordial

Inma