We had some discussion and clarification of the rules and I think we need a summary to make everything clear. This is my attempt, but if other players and Magmaniac can check and correct it then we can have an official rules post.
Rules for Lord of the Rings Diplomacy
1.1 Same rules as standard Diplomacy, except for as below.
Fleets
2.1 There are two types of fleets, empty and full.
2.2 Both types of fleet may make a downstream (double) move or a normal move.
Full Fleets
3.1 Full fleets cannot convoy
3.2 The army in a full fleet may disembark leaving an empty fleet. The fleet may be in any type of province (river, coastal or sea). The army is ordered to move from the fleet. Example: The fleet is full and in Aaa, you write the order as A Aaa -> Bbb. The army will move as an army normally moves, and the fleet will hold. If another unit moves to Aaa, the army moving away is not affected, but the fleet is. Note then that the fleet will now be an empty fleet.
3.3 When an army leaves a fleet, the empty fleet left behind may do a convoy on the same turn.
3.4 If a full fleet is dislodged, then the army alone may retreat leaving an empty fleet. If the incoming unit is an army then it captures the empty fleet and becomes a full fleet. If the incoming unit is a full fleet, then the empty fleet remaining is destroyed.
Empty Fleets
4.1 Empty fleets have 0.5 power. They are dislodged if one full fleet moves to the same space as it.
4.2 Empty fleets cannot: support, cut support, capture supply centres or dislodge units.
4.3 Empty fleets can convoy.
4.4 Empty fleets do not use supply.
4.5 An empty fleet may be supported to hold giving it power 1.5, so that it is not dislodged by an attacking fleet or captured by an attacking army (unless they have support).
4.6 If an empty fleet moves to a space at the same time as a full fleet or an army, the full fleet or army successfully reaches its destination and the empty fleet bounces.
4.7 An empty fleet may be supported to move into a province. If another empty fleet attempts to move to the same province, the fleet with the most support will succeed, otherwise they will both bounce.
4.8 If an army moves to the same province as an empty fleet that has not moved (because it was ordered to hold or because it bounced) it captures the empty fleet and becomes a full fleet.
4.9 An army may move to a river or coastal province to capture/load into a fleet. (not a sea province)
Movement
5.1 A Bridge or Mountain Pass denoted by a gray line can be used by armies but not fleets.
5.2 It is not possible to move from the Falls of Rauros (FORA) to space 33, the black line separating these spaces is not a bridge and shows this area is impassable.
5.3 The coastlines of Sarn Gebir (Sag) and Tol Brandir (Tob) are not separated by the Falls of Rauros (FORA). A fleet in either of these provinces may move to any river, coast or sea province adjacent to it.
Fleet Downstream Move
6.1 A fleet may move in a downstream direction and move two spaces in one turn.
6.2 The middle province in the move must be a river province, denoted by a number on the map, the start and end provinces may be coast, sea or river provinces.
6.3 Downstream direction is shown on the map by the province numbers increasing. e.g. 27 is downstream from 26.
6.4 If both the start and end provinces are coastal then they must be on the same side of the river.
6.5 To resolve the downstream move, first all units make a single move, which is resolved in the usual way. Then fleets make a second move, which is resolved. This may lead to situations not found in standard Diplomacy e.g. a fleet dislodging two units in a turn, two units being dislodged in the same province, a unit ending up alone in a province and being dislodged or a unit moving to a province and being dislodged in the same turn.
Building
7.1 A player can build on any supply center they control.
7.2 Only one Fleet can be built per year by a power, except for the Gray Havens, Rhun, or Umbar, who can each build two.
7.3 An empty fleet can be built during the build phase as long as the power does not have to disband units due to loss of supply,
7.4 An empty fleet can be built on a center already holding an army, so that it becomes a full fleet.
7.5 A full fleet may be built in a supply centre. This would count towards the limit for fleet building and the normal supply limit.
7.6 A supply centre may not be built on in the same year it was taken control of. e.g. if an army captures a supply centre, an empty fleet may not be built on it in the following Winter turn.
Convoying
8.1 Only empty fleets can convoy, full fleets cannot.
8.2 If an army tries to board and empty fleet that is convoying an army, the result is a stand off. The convoy fails and the army fails to capture the fleet.
8.3 A convoy has to be from land to land. If there is an empty fleet in the land destination, it creates a full fleet there. If an army is convoyed from departing a fleet on a land space, it leaves the empty fleet behind there. You can't convoy from one fleet to another.
Edit: It's been approved by Magmaniac, so it's official now.