First of all-- you should check out a guidebook on Egypt, I'm sure they will have info on how to get there by boat.
This is what I found online-- Adriatic Lines operates ferry service to Alexandria & Port Said from Venice and also Greece; I would recommend going from Greece because it would only be one day instead of three (from Venice). Here is what I found and I'll post more if I come across it:
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By Sea
Alexandria and Port Said on the Mediterranean Sea, and Suez and Nuweiba on the Red Sea are ports of entry for visitors, but sailings have been reduced of late. Check with your travel agent or the following operators:
Adriatic Lines, Castro and Company, 12 Talaat Harb. Tel: 743-213, 743-144 (passengers and shipping).
The Egyptian Navigation Company, 26 Sherif. Tel: 393-8278; 1 Huria, Alexandria. Tel: 472-0824.
Favia Shipping Lines, 18 Adli. Tel: 393-8983.
Federal Arab Maritime, 27 Gazirah el Wosta, Zamalek. Tel: 341-5823, 340-6351.
The International Agency for Tourism, Navigation and Trading Services, 13 Midan Tahrir. Tel: 62-892, 779-452.
International Transport & Maritime Service Co., 6A Asma Fahmi, Kulliet el Banat, Heliopolis. Tel: 661-783.
Misr Edco Shipping Company, Menatours, 14 Talaat Harb. Tel: 776-951.
North African Tourist Shipping, 171 Muhammad Farid. Tel: 391-3081, 391-4682, or el Takkadom, Madinat Nasr. Tel: 608-417 ( only from Port Said to Cyprus and Haifia).