What Is the `Awrah of a Man?
Question

Is the extent of the `awrah from the navel to the knee or from beneath the navel to above the knee? If it is the latter, is it permissible for me to go to the sea wearing shorts without a t-shirt on a mixed beach or not? Please note that my navel will be showing and the shorts end a little above the knee. May Allah reward you with good!
Praise be to Allah.`Awrah of a man
The `awrah of a man is the area between the navel and the knee; as for the navel and the knee themselves, they are not part of the `awrah. 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: 
“There are several opinions concerning the matter: 
One is that the knee is included in the `awrah so it must be covered.
The second opinion is that the navel and the knee are both part of the `awrah so they must both be covered.
The third opinion – which is the well-known view among our madhhab – is that the navel and the knee are not included in the `awrah, so they do not have to be covered. This is based on the definition of the `awrah as being “between the navel and the knee .” (Ash-Sharh al-Mumti‘, 2/160) 
He also said (may Allah have mercy on him): “The `awrah for a man is the area between the navel and the knee.” (Ash-Sharh al-Mumti‘, 5/273) 
An-Nawawi (may Allah have mercy on him) said:

Concerning the `awrah for men , there are five opinions; the correct view is that it is the area between the navel and the knee, and the navel and the knee are not part of the `awrah. 
Our evidence for that is what we have quoted above from Abu’d-Darda (may Allah be pleased with him) who said: I was sitting with the Prophet (blessings and peace of Allah be upon him) when Abu Bakr (may Allah be pleased with him) came, holding up the hem of his garment to such an extent that his knees showed. The Prophet (blessings and peace of Allah be upon him) said: “Your companion has had a quarrel.” And he quoted the hadith. (Narrated by al-Bukhari) 
Abu Musa (may Allah be pleased with him) narrated that the Prophet (blessings and peace of Allah be upon him) was sitting in a place where there was water and his knee or knees were uncovered, then when ‘Uthman came in, he covered them. Narrated by al-Bukhari. And we have quoted above hadiths which speak of the thigh being `awrah .” (Sharh al-Muhadhdhab, 3/174, 175) 
For the rest of the evidence please see the answer to question no. 34976  
And he said in ash-Sharh al-Kabir, 1/457: “The navel and the knees are not part of the `awrah. This is the view of Malik and ash-Shafi'i.” 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: 
“It is not permissible for a young man to play football, for example, wearing only shorts that do not reach the knees or that do not reach the navel. It is essential that the area between the navel and the knee be covered in the case of young men.” (Liqa’ al-Bab al-Maftuh, no. 73) 
Can you go to the sea wearing clothing that covers the area between the navel and the knee?
Based on that, it is permissible to go to the sea wearing clothing that covers the area between the navel and the knee. But if it leaves any part of the thigh or anything lower than the navel uncovered, that is not permissible. 
But if that is in a place where there is mixing, it is not permissible because that involves staying in a place where evil actions are being committed when one is not able to change them or denounce them. 
And Allah knows best.