How can I combine this group of sentences into a single sentence?
A storm came up. Gilligan's craft sank in three feet of water in the lagoon. The skipper cried bitter tears over the loss of his little buddy.
There are certainly several ways in which one may combine simple sentences into fewer or even one sentence. One way to join ideas is to subordinate the less important one(s) to the main idea. This is often a good idea, too, because in this way the reader understands what is truly important. Now, regarding the sentences under question, the most important idea is that the boat of Gilligan sank. In these three sentences there is also the result of the sinking as well as the time and conditions in which it happened. So, the best way to combine these ideas is to use an adverbial phrase to indicate the time and conditions which came first:
1. When the storm came up, OR After the storm came up,
Then, make an independent clause of the most important idea:
2. Gilligan's craft sank in three feet of water in the lagoon
Finally,establish the relationship of cause and effect between the boat's sinking and Gilligan with the appropriate word. Here, you can use a participial phrase to make a dependent clasue, or you can use another independent clause if you think this idea is as important as the sinking of the boat. Here are examples for each:
(a) [participial phrase] ...lagoon, causing him to cry bitter tears over the loss of his "little buddy."
(b) [another independent clause] ...lagoon; consequently, he cried bitter tears over the loss of his "little buddy."