What was the reason for the Sultanate to accept Ottoman suzereinty? Also, I remember reading about a sultanate in Indonesia that became vassals of the Ottomans, too. What was it?
As I am currently away from home for work I don't have access to my books. If you are interested in Ottoman activity in the Indian Ocean Giancarlo Casale's The Ottoman Age of Exploration is definitely the go-to source and a quite accessible read. I will also refer you to two older posts of mine (1 and 2) which deal with the topic and rely upon Casale's work.
The Indonesian sultanate you are thinking of is the Sultanate of Aceh, a maritime oriented Muslim state on the northern tip of the island of Sumatra and eponymous to the modern Indonesian province named Aceh. In the mid-16th century the relatively young Sultanate reached out to the Ottoman Empire, reigned by Suleiman the Magnificient at the time, and accepted the suzerainty of the far away caliph in Constantinople. In exchange for the declared allegiance the Ottomans sent a detachment of Turkish gunners with artillery and ships to Sumatra to assist Aceh in her fight with local rivals and, most importantly, the Portuguese-controlled Malacca.
Seemingly, all the Acehnese had to do get this support was to accept the theoretical overlordship of the distant Ottoman caliph and to pay respects by reading out the khutbah, the sermon at the friday prayer, in Suleiman's name. Seems like a good enough deal to me. So what do the Ottomans get out of this? Well, over the course of the 16th century the Ottomans were quite eager to assert their influence in Muslim states all around the Indian Ocean's shores–especially reaching out to those rulers who opposed Portuguese influence in their domains. The presence of Christian warships, pirates, and traders (with no clear distinction between all of those) in the profitable Indian Ocean trade network was a thorn in the Ottomans' side. Plus, a Christian presence on the other side of the empire meant a continuous threat to the Ottoman-controlled Red Sea and thereby the Holy Cities of Mecca and Medinah.
This was the reasoning behind the Ottomans propping up smaller Muslim states in the Indian Ocean world. Muslim rulers in South and South East Asia, and East Africa on the other hand could gain a valuable ally supplying knowledge and critical elite troops to their advantage.
Unfortunately I don't know anything about the Maledives in particular, but I take an educated guess that the case was quite similar to that of Aceh or Gujarat in India.