The Sea to Sea (Yam el Yam) trail is approximately 55 miles (89 kilometers), stretching from the Mediterranean in the west to the Sea of Galilee in the east. The total elevation gain/loss is 5,250 feet (1600 meters). Though the high-90s temperatures in mid-September thwarted my attempt to walk across Israel in 3 days, I managed to hike 22 miles in one day, including climbing 2 of the tallest peaks in Israel, before concluding that it would be better to be sane and quit the death march.
You must have the Society for Protection of Nature in Israel (SPNI) Hiking Map #2 (see above left images-- click to enlarge them), which is only available in Hebrew and is sold at main SPNI offices and some major bookstores in Tel Aviv. The interconnected trails are well marked by the SPNI staff, and for the second half of the hike you will just follow the orange/blue/white striped marks (see above right image).
There are occasional faucets along the trail (these are marked on the SPNI map), and villages and towns close to the trail. However, you should plan on carrying at least a couple of liters at any given time. The last section of the trail, leading to the Sea of Galilee, has a dry and hot canyon passageway (try to reach intersection with Hukuk/Livnim by 10am) and a steep, narrow, and dangerous descent.
Accommodations-wise, most walkers are part of youth groups that camp along the way (see More Information below for campsites; SPNI's map also identifies them). It's possible to avoid carrying a tent and stay in shelters, though the B&B/zimmer/hotel accommodations are usually off the trail and are hard to find using the SPNI map, so factor that into your daily mileage. If you're looking for a roof and a bed, try to plan your walk around the following towns/villages: Nahariya (I stayed at Hotel Frank, which was fine); Abirim or Fasuta; Meron; Safed (also spelled Tsfat, etc., though be forewarned that it is on the third highest peak in Israel and a long way up from the valley by foot!); etc... see the SPNI map for dense areas and aim towards those. SPNI provides trail angels and Field Schoolsthat offer accommodation, though few are on the trail route. The Mt. Meron Field School is just off the trail-- though be forewarned it feels a bit like desolate, dilapidated former military barracks.