Apart from the White Ship in 1120, have there been any other maritime disasters in which an heir to a throne and/or many notable people died?

I would suggest as another possibility not an heir to a throne, but a mighty monarch who might have died with thousands of others, some of them probably important people, in a sea disaster.

Musa II, Mansa of the Malian Empire, or Mansa Musa, is famous as one of the richest men in history. He ruled from about 1312 to 1337, and his pilgrimage to Mecca in 1324 and 1325 with an immense retinue and spending vast amounts of gold dust is legendary.

Apparently Musa II became Mansa by being selected as the deputy of the previous Mansa when the previous Mansa, Abu Bekr II, went on a journey. The story that Musa II is said to have told in Cairo is:

The ruler who preceded me did not believe that it was impossible to reach the extremity of the ocean that encircles the earth (meaning Atlantic), and wanted to reach that (end) and obstinately persisted in the design. So he equipped two hundred boats full of men, as many others full of gold, water and victuals sufficient enough for several years. He ordered the chief (admiral) not to return until they had reached the extremity of the ocean, or if they had exhausted the provisions and the water. They set out. Their absence extended over a long period, and, at last, only one boat returned. On our questioning, the captain said: 'Prince, we have navigated for a long time, until we saw in the midst of the ocean as if a big river was flowing violently. My boat was the last one; others were ahead of me. As soon as any of them reached this place, it drowned in the whirlpool and never came out. I sailed backwards to escape this current.' But the Sultan would not believe him. He ordered two thousand boats to be equipped for him and for his men, and one thousand more for water and victuals. Then he conferred on me the regency during his absence, and departed with his men on the ocean trip, never to return nor to give a sign of life.[7]

https://en.wikipedia.org/wiki/Abu_Bakr_II1

Mohammed Hamidullah. "Echos of What Lies Behind the 'Ocean of Fogs' in Muslim Historical Narratives". Muslim Heritage. Retrieved 27 June 2015. (Quoting from Al-Umari 1927, q.v.)2

So possibly the mighty ruler Abu Bekr II of Mali died in a sea disaster, which might be considered even more important that the deaths of Margaret the Maid of Norway, Prince William of England, or Jan William Friso.

Apart from the example of Margaret, Maid of Norway cited in Mark's answer, there seems to be only one other possible heir: John William Friso, Prince of Orange.

John William Friso, according to the Wiki article on the The Second Stadtholderless Period, was made William III's heir in 1702 according to his last will and testament:

As William died without (legitimate) issue, he had to make provisions in his last will and testament to prevent any uncertainty. Indeed, he made John William Friso, Prince of Orange, head of the cadet branch of Nassau-Dietz of the family, his general heir, both privately and politically.

Note: Although the article does not state it, this did not include the Kingdom of England.

Unfortunately for Friso, five of the seven Dutch provinces did not follow William III's will so his

rejection as stadtholder by five of the seven Dutch provinces in 1702 marked the return to political supremacy of the States General

It appears that all was not lost for Friso, though, as he became a general under the Duke of Marlborough and served with distinction until he drowned when a ferry he was on sank in bad weather in July 1711. This seems to have been particularly bad timing as (from Wiki)

The prestige that he acquired from his military service should have favored his eventual elevation as stadtholder in the remaining five provinces.

At the time of his death Friso was, technically at least, still William III's heir. His posthumous son, William IV, was born six weeks later and, in 1747 became the first hereditary stadtholder of all the provinces.