If there are differences in his postion and theirs, I suspect that he will do what he did in 2008 as a surrogate for Obama, when he disagreed on the energy bill. He gave Obama's position as fairly as he could giving the best arguments for it, then when asked gave his own opposite opinion and the reason he was for that. Now, he is not a surrogate for Obama, but an ally and a loyal Democrat (which sadly others were not for him in the short time when he was the titular head of the party.)
I suspect that he will emphasize where they agree. The Obama administration and Kerry all strongly back a 2 state solution and some have spoken of the blockade as untenable. In reality the best thing that Obama/Clinton could have done in 2009 was to question the blockade more than they did. Kerry is still the highest level official who went to the West Bank. He questioned the blockade then and brought up the issue that they were excluding pasta with the Israelis and got that changed.
Kerry in many public speeches has taken a very balanced position. He has often spoken of seeing young children playing in the ruins of what were buildings and has spoken of the lack of freedom of movement of West bank Palestinians. His speech before AIPAC went further than Biden's in challenging AIPAC's positions. His voice matters in the Middle East, because he is seen as honest by the moderate Arabs. Other than Mitchell, I'm not sure anyone else from the US is - other than hopefully the President. Between his ability to meet with Syria's President, his wonderful Doha speech (
http://acommonword.com/en/a-common-word/11-new-fruits-of-a-common-word/395-text-of-senator-john-kerrys-remarks-at-the-us-islamic-world-forum-in-doha.html ) and his very down to earth, straight comments at the Jordan financial forum (
http://www.democraticunderground.com/discuss/duboard.php?az=show_mesg&forum=273&topic_id=157426&mesg_id=157426) This is a credit that Kerry worked hard to earn and it is valuable to the country. I seriously doubt he will throw that away.
His own statement was not that far from where the administration was at that point - "I am saddened by the tragic loss of life in the waters off Gaza. At this point, it is unclear what happened and there must be a thorough investigation. This unfortunate incident underscores the necessity of resolving the conflict between Israel and the Palestinians."
http://kerry.senate.gov/cfm/record.cfm?id=325362 I suspect that Kerry will speak both of plight of the people of Gaza and the real security threats to Israel - he has mentioned many times of speaking in Gaza of the fact that no country could tolerate its neighbor shooting rockets into it - even using Quincy and Boston as an example. I suspect that he will speak of the necessity to prevent weapons from entering the Gaza strip as a valid Israeli concern. I hope he will speak of the fact that the blockade is greatly harming the people. I would guess that he has been internally pushed and may be why they are calling the blockade untenable.
(I suspect that people on DU will not be happy as he will take a more moderate, balanced position than they like.)
This really is a PR nightmare for Israel. It is clear that overall goal of the people on the boat was to end the blockade. By responding as Israel did, it has put a spotlight on the blockade - and it is hard to defend.
I posted the J Street response on DU P,
http://www.democraticunderground.com/discuss/duboard.php?az=show_topic&forum=389&topic_id=8460947 Their position is where I would like the administration to be.